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insects
Review
Diversity of Termite Breeding Systems
Edward L. Vargo
Department of Entomology, Texas A&M University, College Station, TX 77843, USA; ed.vargo@tamu.edu;
Tel.: +1-979-845-5855
Received: 14 December 2018; Accepted: 30 January 2019; Published: 12 February 2019
Abstract:
Termites are social insects that live in colonies headed by reproductive castes. The breeding
system is defined by the number of reproductive individuals in a colony and the castes to which
they belong. There is tremendous variation in the breeding system of termites both within and
among species. The current state of our understanding of termite breeding systems is reviewed.
Most termite colonies are founded by a primary (alate-derived) king and queen who mate and
produce the other colony members. In some species, colonies continue throughout their life span
as simple families headed by the original king and queen. In others, the primary king and queen
are replaced by numerous neotenic (nymph- or worker-derived) reproductives, or less commonly
primary reproductives, that are descendants of the original founding pair leading to inbreeding in
the colony. In still others, colonies can have multiple unrelated reproductives due to either founding
the colonies as groups or through colony fusion. More recently, parthenogenetic reproduction has
shown to be important in some termite species and may be widespread. A major challenge in termite
biology is to understand the ecological and evolutionary factors driving the variation in termite
breeding systems.
Keywords: reproductives; king; queen; neotenics; parthenogenesis
1. Introduction
Like all eusocial insects, termites have a reproductive division of labor in which specialized castes
reproduce and other members of the colony work on their behalf. For the purposes of this review,
the breeding system is defined as the number and caste members engaged in reproduction within
termite colonies. Here, breeding system and breeding structure are used interchangeably. The breeding
system determines the degree of genetic relatedness among colony members, as well as the level of
inbreeding of individuals within the colony. Therefore, understanding the breeding system of a species
is critical for knowing both the direct and indirect fitness benefits that colony members experience.
The colonies of most termites are started by a pair of primary (alate-derived) reproductives
following a mating flight. The primary reproductives, the king and queen, form a monogamous
pair that produces the other colony members. In many termites, especially in the so-called lower
termites, but also in some higher termites, neotenic individuals (derived from nymphs or workers),
assume the reproductive role, usually after the death of the king and/or queen. Of course, there
are many exceptions to this general rule, and recent studies combining field censusing and genetic
analyses continue to add new twists to the breeding system of termite colonies. The breeding system
of termites can be categorized as simple families, extended families, mixed families, or involving
parthenogenetic reproduction.
Termite breeding systems have been the subject of a couple of reviews. One by Vargo and
Husseneder [
1
] focused on the subterranean genera, Coptotermes and Reticulitermes, and another
focused on the colony and population genetics of termites [
2
]. Since these reviews, there have been
some important developments in our understanding of termite breeding systems. In this review, I will
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Insects 2019,10, 52 2 of 12
present the current state of knowledge in the field. First, I review the various reproductive castes found
in termites.
2. Reproductive Castes
There are several different castes in termites capable of reproduction. Not all castes are present in
all species.
2.1. Primary Reproductive
The main reproductive castes are the primary male and female reproductives. These are the
only true adults in the colony, with fully developed wings, compound eyes, and body pigmentation.
Primary reproductives fly from the colony and then land at varying distance from the natal nest. Males
(kings) and females (queens) pair up to form what is known as tandem pairs, then locate a nest site,
mate and produce offspring. Primary reproductives are the main way that new colonies are formed.
2.2. Neotenic Reproductive
Neotenic reproductives develop from either nymphs or workers. Nymph-derived neotenics,
known as brachypterous neotenics, have wing pads and often are slightly melanized. Worker-derived
neotenics, called apterous neotenics, lack wing pads and show little if any melanization. The occurrence
of neotenics throughout the termites is reviewed by Myles [
3
]. Neotenics are widespread among the
lower termites where they occur in over 60% of the genera, whereas they occur in only about 13% of the
higher termite (Termitidae) genera. Since neotenics are unable to fly, they stay in the nest where they
inherit existing resources, including the colony worker force. They allow the colony to live beyond the
lifespan of the primary reproductives but they inbreed within the nest. Neotenics tend to be less fecund
than primary reproductives, but they often occur in large numbers and the combined reproductive
output of the group can far exceed that of the primary reproductives. Neotenics are common in wood
feeding termites but are rare in mound building and humivorous species.
2.3. Soldier Reproductive
In some lower termites, individuals of both sexes with soldier-like morphology have functional
gonads and reproduce [
4
]. This occurs in four species of Archotermopsidae and two species of
Stolotermitidae, but are especially common in Archotermopsis wroughtoni and Zootermopsis nevadensis
[5,6]
.
3. Types of Breeding Systems
3.1. Simple Families
The colonies of most termites, at least during the founding stage and on through the early colony
growth phase, are headed by a single pair of monogamous primary reproductives. Such colonies,
in which colony members are the progeny of one king and one queen, are simple families. In some
higher termites, colonies remain simple families throughout their life cycle. However, in many termites,
including some higher termites and most lower termites, the breeding structure can change from a
simple family through the addition of new reproductives, usually neotenics, or through colony fusion.
Whether colonies change from simple families to some other breeding system and the point in the
lifecycle when the change occurs differs tremendously among species. This variation within and
among species underlies much of the diversity of breeding systems in termites. The selective forces
favoring one breeding system over another are not well understood.
While most simple families are headed by primary reproductives, this may not always be the
case. It is possible for a colony to be a simple family headed by two neotenic reproductives or one
neotenic and one primary reproductive, as long as all the individuals in the colony are the offspring
of the reproductive pair. Since neotenics cannot found colonies independently, for a simple family to
be headed by one or two neotenics, the neotenics would need to reproduce for a long enough period
Insects 2019,10, 52 3 of 12
that their offspring replace any offspring of the primary reproductive(s) they supersede. Given the
relatively long life span of many termite workers and soldiers, such situations are most likely rare.
The proportion of colonies forming simple families within a population is highly variable both
within and among species. For example, only simple family colonies were found in 17 colonies
studied genetically in the Neotropical termitid Labiotermes labralis [
7
] and 38 colonies of the Australian
subterranean termite Coptotermes lacteus [
8
], whereas about half of 13 colonies of the subterranean
termite R. malletei were simple families and half were extended families [9].
In the case that a simple family is headed by a king and queen who are unrelated to each other, the
colony will be outbred. In contrast, colonies headed by siblings will be inbred. The only way to tell the
degree of relatedness of the reproductives is by genotyping them directly, or more commonly, inferring
their genotypes from the genotypes of workers. Vargo and Husseneder [
2
] reviewed the studies that
have inferred the degree of relatedness among the reproductives heading simple family colonies.
There is considerable variation within and among species in the degree of relatedness between kings
and queens. The reasons for this variation are unknown but are presumably related to the distance
dispersed by alates from the natal colony. Where alates fly long distance, the chance that siblings pair
up should be low, whereas short-range dispersers are more likely to pair with siblings.
The use of genetic markers can uncover breeding systems that may be difficult to obtain through
field censuses. For example, in the most basal termite, Mastotermes darwiniensis (Mastotermitidae),
the colonies are founded by primary reproductives, but only numerous neotenics have been found in
excavations of established field colonies [
10
]. Goodisman and Crozier [
11
] conducted genetic analysis
of 18 colonies of M. darwiniesis in northern Australia using microsatellite markers. They found that
26% of the colonies had genotypes consistent with a single monogamous pair of reproductives, while
47% had genotypes characteristic of multiple inbreeding neotenics derived from a monogamous pair
of reproductives, and 27% had evidence of complex genotypes indicating the presence of offspring
descended from multiple primary pairs, either from colonies founded by multiple pairs of primary
reproductives or the fusion of colonies after establishment. Thus, the breeding system of M. darwiniensis
is much more complicated when viewed through the lens of genetic data than through that of
field collections.
3.2. Extended Families
Colonies headed by multiple secondary reproductives descended from a single monogamous
pair are called extended families following the terminology of Vargo [
12
]. Most often, such colonies
are headed by multiple neotenics (either worker or nymph-derived reproductives) that are descended
from the primary king and queen. Neotenics are more common in the lower termites [
3
], so extended
family colonies are expected to be more common in these taxa than in the Termitidae. However, in
some termitids, secondary reproductives can be alate-derived primary reproductives recruited from
within the colony [13], so extended family colonies are not restricted to the presence of neotenics.
Extended family colonies can be identified genetically using biparentally inherited markers such
as microsatellites or allozymes. The genotypes of a pool of workers in extended family colonies have no
more than four alleles at a locus, the maximum number possible from a mating of two individuals but
have the presence of more genotypes than possible from a mating between a monogamous pair (e.g.,
an allele paired with itself and two other alleles). Examination of a mitochondrial DNA marker would
show that all workers were identical since they all descended from the same original foundress queen.
By definition, extended family colonies are headed by interbreeding groups of closely related
individuals leading to inbred colonies. The degree of inbreeding within a colony will depend on the
number of breeders and the number of generations of inbreeding. In general, the fewer the numbers of
breeders and the more generations they inbreed, the more inbred the colony will be. If there are many
extended family colonies in a population, this can have a strong effect on the overall level of inbreeding
in the population. In the genus Reticulitermes, the proportion of extended family colonies can vary
from 5% in a population of R. hageni [
14
] to 100% in invasive populations of R. flavipes [
15
]. Within a
Insects 2019,10, 52 4 of 12
species, there can be considerable variation in breeding system types. Figure 1shows variation in the
breeding system among populations of R. flavipes in the U.S.
Neotenics are produced in colonies in response to orphaning, i.e., the death of the primary king or
queen. Recent work by Sun et al. [
16
] shows that there is both sex-specific inhibition and stimulation
of neotenic development in R. flavipes. These authors show that the presence of neotenics of one
sex inhibits workers of the same sex from developing into neotenics but also stimulates workers of
the opposite sex to differentiate into neotenics. The production of neotenics has long been believed
to be under the influence of pheromones [
17
], but it was not until recently that biologically active
compounds were identified. Matsuura et al. [
18
] identified two volatile compounds, n-butyl-n-butyrate
and 2-methyl-1-butanol, that are the active components in the queen pheromone that inhibits the
development of female neotenics in R. speratus. These compounds are also produced by eggs [
18
]
and inhibit egg laying among female neotenics [19]. So far, this is the only pheromone identified that
inhibits the development of female neotenics and no pheromones have been identified that inhibit
the development of male neotenics. Hayashi et al. [
20
] proposed a sex-linked genetic mechanism
underlying the production of different castes in R. speratus, including primary reproductives and both
brachypterous and apterous neotenics. While this mechanism seems to apply under controlled rearing
conditions in the lab, its predictions do not hold up in field colonies [
21
]. Thus, the ability of this model
to explain neotenic production in the field is suspect.
Insects 2019, 10, Firstpage-Lastpage; doi: www.mdpi.com/journal/insects
number of breeders and the number of generations of inbreeding. In general, the fewer the numbers
of breeders and the more generations they inbreed, the more inbred the colony will be. If there are
many extended family colonies in a population, this can have a strong effect on the overall level of
inbreeding in the population. In the genus Reticulitermes, the proportion of extended family colonies
can vary from 5% in a population of R. hageni [14] to 100% in invasive populations of R. flavipes [15].
Within a species, there can be considerable variation in breeding system types. Figure 1 shows
variation in the breeding system among populations of R. flavipes in the U.S.
Neotenics are produced in colonies in response to orphaning, i.e., the death of the primary king
or queen. Recent work by Sun et al. [16] shows that there is both sex-specific inhibition and
stimulation of neotenic development in R. flavipes. These authors show that the presence of neotenics
of one sex inhibits workers of the same sex from developing into neotenics but also stimulates
workers of the opposite sex to differentiate into neotenics. The production of neotenics has long been
believed to be under the influence of pheromones [17], but it was not until recently that biologically
active compounds were identified. Matsuura et al. [18] identified two volatile compounds, n-butyl-
n-butyrate and 2-methyl-1-butanol, that are the active components in the queen pheromone that
inhibits the development of female neotenics in R. speratus. These compounds are also produced by
eggs [18] and inhibit egg laying among female neotenics [19]. So far, this is the only pheromone
identified that inhibits the development of female neotenics and no pheromones have been identified
that inhibit the development of male neotenics. Hayashi et al. [20] proposed a sex-linked genetic
mechanism underlying the production of different castes in R. speratus, including primary
reproductives and both brachypterous and apterous neotenics. While this mechanism seems to apply
under controlled rearing conditions in the lab, its predictions do not hold up in field colonies [21].
Thus, the ability of this model to explain neotenic production in the field is suspect.
Figure 1. Variation in the breeding system in populations of Reticulitermes flavipes in the U.S. as
determined by genetic markers.
1
Data from DeHeer and Kamble [51];
2
data from Bulmer et al. [52];
all other data from Vargo et al. [50]. Sample sizes were between 7 and 314 colonies per population.
Recently, Matsuura et al. [21] proposed a genomic imprinting model to explain differentiation
of the reproductive caste in termites, including R. speratus. According to the model, both genetic
imprinting and environmental factors (e.g., the presence of inhibitory pheromones) influence the
differentiation of reproductive castes. The model also predicts that the relative strength of maternal
and paternal imprinting varies among species and accounts for the differentiation of both sexually
produced neotenics and asexually produced neotenics (discussed below). This model, while
appealing, requires confirmation, especially regarding the evidence of sex-specific epimarks. Clearly,
our understanding of the proximate factors underlying neotenic differentiation in termite colonies is
far from complete and needs much additional research.
Figure 1.
Variation in the breeding system in populations of Reticulitermes flavipes in the U.S. as
determined by genetic markers.
1
Data from DeHeer and Kamble [
22
];
2
data from Bulmer et al. [
23
];
all other data from Vargo et al. [24]. Sample sizes were between 7 and 314 colonies per population.
Recently, Matsuura et al. [
21
] proposed a genomic imprinting model to explain differentiation
of the reproductive caste in termites, including R. speratus. According to the model, both genetic
imprinting and environmental factors (e.g., the presence of inhibitory pheromones) influence the
differentiation of reproductive castes. The model also predicts that the relative strength of maternal and
paternal imprinting varies among species and accounts for the differentiation of both sexually produced
neotenics and asexually produced neotenics (discussed below). This model, while appealing, requires
confirmation, especially regarding the evidence of sex-specific epimarks. Clearly, our understanding
of the proximate factors underlying neotenic differentiation in termite colonies is far from complete
and needs much additional research.
Over the last decade, there have been increasing reports of a new type of extended families known
as asexual queen succession (AQS). Under AQS, the founding primary queen produces neotenic
queens parthenogenetically. The parthenogenetic daughters mate with the primary king to which
they are unrelated, or less commonly with a neotenic brother, to produce workers, soldiers and
alates through normal sexual reproduction. This unique breeding system was first reported by
Insects 2019,10, 52 5 of 12
Matsuura et al.
[
25
] in the subterranean termite R. speratus and has since been reported in two other
species of Reticulitermes [
26
,
27
] as well as three species of higher termites [
28
,
29
]. AQS is discussed in
more detail below, under Parthenogenetic Reproduction.
3.3. Mixed Family Colonies
Colonies in which the offspring show more complex genotypes than is possible if they are all
descended from a primary reproductive pair are called mixed families. These can form either at the
founding stage if multiple same sex reproductives cooperate to found colonies or later in the colony
life cycle by the merging of two or more colonies. Such colonies will have five or more alleles at one or
more nuclear loci and may have multiple mitochondrial DNA haplotypes present if there are two or
more unrelated female reproductives present.
Mixed family colonies occur frequently in some groups of termites. Goodisman and Crozier [
11
]
found that 5 of 19 (26%) M. darwinienisis colonies genotyped were mixed families. The colonies of
Zootermopsis nevadensis (Archotermopsidae) undergo frequent colony fusion early in their life cycle.
This process and the conditions leading up to it are described in detail by Thorne et al. [
6
,
30
] and
Johns et al.
[
31
]. This species founds colonies through large synchronous mating flights in which
kings and queens pair up and take residence in large downed trees. The presence of many founding
pairs in the same wood resource leads to high densities with closely packed nests. Interactions
among neighboring incipient colonies are common and often result in the death of one of the primary
reproductives and subsequent merger of the interacting colonies. Individuals of the merged colonies
cooperate and function as a single colony. Colonies that engage in aggressive interactions often
generate neotenic reproductives as well as reproductive soldiers. These reproductive soldiers are only
aggressive against non-nestmate reproductives. Genetic studies in the lab show that reproductives
from different colonies can coexist and interbreed to form mixed family colonies, and studies of
mature field colonies also found the presence of mixed family colonies in this species [
32
]. These
observations led Thorne et al. [
6
] to propose the “accelerated inheritance” hypothesis for the evolution
of eusociality in termites. This hypothesis proposes that nonreproductive helpers (workers) that stay
in the nest to assist their parents have a better chance of developing into a reproductive and inheriting
the nest following aggressive encounters with conspecific colonies than they have of founding their
own colony independently. These authors also hypothesize that soldiers evolved in termite colonies
as an aggressive reproductive caste first and then assumed the role of sterile colony defenders later.
Evidence of reproductive soldiers in other taxa of basal termites beyond the six species where they
have been found would lend more credence to this idea.
Mixed family colonies have been found in the drywood termite, Cryptotermes secundus
(Kalotermitidae), where they make up about 25% of all mature colonies studied [
33
]. These
colonies form from the fusion of conspecific colonies when nest density is high in the same wood
resource. Mixed families have been reported from subterranean termites (Rhinotermitidae) where
they presumably occur through colony fusion [
1
,
34
]. Mixed family colonies of higher termites have
also been reported. Multiple reproductives sometimes cooperate to found the colonies of Macrotermes
michaelseni and maintain stable associations throughout the colony’s life cycle [
35
–
37
]. The colonies
of Nasutitermes corniger often have multiple primary reproductives [
13
,
38
,
39
]. Studies of populations
of N. corniger in Panama indicate that unrelated primary reproductives can arise through multiple
primary reproductives associating during colony foundation, colony fusion and adoption of unrelated
reproductives into established nests [
13
,
39
]. Thus, this one species forms mixed family colonies through
all possible routes.
In the higher termites, mixed family colonies do not always involve primary reproductives.
Haifig et al. [
40
] found unrelated neotenic reproductives co-existing in a colony of Silvestritermes
euamignathus. Using mitochondrial DNA and microsatellite markers, these authors showed that the
59 neotenic reproductives found in the same nest descended from at least four different reproductive
Insects 2019,10, 52 6 of 12
pairs representing at least two maternal lineages. Haifig et al. [
40
] proposed that these neotenics either
descended from a group of cooperating primary reproductives or were the result of colony fusion.
The ultimate and proximate factors favoring colony fusion are not well understood. In theory,
colony fusion should be selected against since it will decrease the genetic relatedness among nestmates
and, therefore, decrease indirect fitness benefits that individuals might gain through helping kin
reproduce. In fact, genetic studies have shown that the colonies of R. flavipes that fuse together are not
closely related based on nuclear microsatellite markers [
34
]. It may be that, in some cases, the cost of
defense and not fusing outweighs the cost of fusing. For example, as discussed earlier, the incipient
colonies of Z. nevadensis can form at very high densities in down trees such that contact with other
incipient colonies is inevitable as colonies grow and expand their nests [
6
,
30
]. In such situations,
it may be that the cost of defense against other colonies becomes higher than the decrease in fitness
due to fusion. Moreover, by merging with nearby colonies, the fused colonies are larger and likely
outcompete other incipient colonies.
In terms of proximate mechanisms underlying colony fusion, there may be a number of factors
involved. There is evidence of a maternally inherited factor in R. flavipes. DeHeer and Vargo [
34
]
documented eight cases of colony fusion in this species, and in all cases, the colonies that fused had
identical or nearly identical mitochondrial DNA haplotypes despite the fact that the neighboring
colonies of this species have very different mtDNA haplotypes and despite the fact that the fused
colonies were unrelated to each other at nuclear markers. The nature of this maternal factor is not
known, but it could possibly be related to microbial symbionts that may be maternally inherited.
For example, Matsuura [
41
] was able to influence the acceptance of foreign workers by manipulating
the gut symbiont community in R. speratus. Certainly, the ultimate and proximate mechanisms
underlying colony fusion in termites warrant much future study.
3.4. Parthenogenetic Reproduction
Thelotokous parthenogenesis has been known to occur in the eusocial Hymenoptera for many
decades. It is most common in the ants where it has evolved at least four times independently and is
present in at least 14 species [
42
]. Since first reported by Light in 1944 [
43
] for the genus Zootermopsis,
the ability to produce parthenogenetic eggs has been reported from some 11 termite species in four
families [
44
]. However, these studies primarily involved laboratory observations of either neotenic
or primary queens kept isolated from males, and Light [
43
] concluded “there is no evidence that the
ability to reproduce parthenogenetically might be of significance in the life of the species”. This all
changed nearly a decade ago with the discovery of asexual queen succession (AQS) in the subterranean
termite R. speratus, a system in which the primary queen is succeeded by parthenogenetically produced
neotenic queens [
25
]. In the AQS system, colonies are founded in the standard way of termites where
a primary king and queen pair up after a mating flight, mate and rear sexually produced offspring.
The primary queen is outlived by the primary king. Before she dies, the primary queen produces a
number of parthenogenetic daughters that develop into brachypterous neotenics who then mate with
the king to produce workers and alates through normal sexual reproduction. Subsequent generations
of neotenics are produced parthenogenetically.
AQS would seem to have many advantages over the usual production of neotenics through
sexual reproduction. First, since the king is unrelated to the neotenic queens, no inbreeding takes
place in the colony as long as the primary king remains alive and reproductively active. Workers
and alates produced in the colony retain their genetic diversity. Second, the number of neotenic
queens can be quite high, as many as 676 [
45
], and their combined reproductive output greatly exceeds
that of a single primary queen. Thus, AQS colonies can grow faster and larger than those headed
by a single monogamous pair of reproductives. From the queen’s perspective, she can continue
to contribute her full complement of genetic material to future generations unlike a queen that is
succeeded by sexually produced neotenics who have only half of her genes. Third, in the cases where
parthenogenesis occurs by automixis with terminal fusion, i.e., in which the second polar nucleus
Insects 2019,10, 52 7 of 12
fuses with the oocyte after the second meiotic division, as in R. speratus [
25
], the parthenogenetic
daughter queens should be nearly homozygous at all loci. In such cases, genetic purging should occur
through the unmasking of lethal recessive alleles in the parthenogens [
45
]. By producing workers and
alates through sexual reproduction, these individuals retain genetic diversity which is advantageous
in dealing with environmental contingencies they may face while foraging or dispersing. Neotenics,
on the other hand, are sheltered and cared for by workers so that they are not disadvantaged by the
lack of genetic diversity.
Since the first report of AQS in 2009, it has been found in five other species, two subterranean
termites, R. virginicus [
26
] and R. lucifugus [
27
], and three higher termites, Embiratermes neotenicus [
28
],
Silvestritemes minutus [
29
], and Cavitermes tuberosus [
46
]. The distribution of AQS within the termites
suggests that it may have arisen independently in all of the known cases. The three Reticulitermes
species all occur on different branches within the genus, corresponding to an Asian branch (R. speratus),
a European branch (R. lucifugus) and a New World branch (R. virginicus) [
47
], and the four termitid
species belong to three different subfamilies. In the end, it may turn out that AQS is more widespread
than we currently know and this may lead us to revise our understanding of its evolutionary origins.
Interestingly, different modes of parthenogenesis are used by the different species with AQS,
leading to different levels of heterozygosity among the parthenogens. The three species of Reticulitermes
undergo automixis with terminal fusion, as discussed above. This results in individuals that are
completely homozygous except for any crossing over that may have occurred before meiosis. Complete
homozygosity should result after several generations. Embiratermes neotenicus undergoes automixis
with central fusion, in which the two central polar nuclei fuse after the second meiotic division to
restore diploidy [
28
]. In this case, females have identical genotypes to the queen, except at loci where
crossing over occurs. This seems to be the mechanism of parthenogenesis in the social Hymenoptera
(bees and ants) where thelotoky has been demonstrated [
42
]. Transition to homozygosity is very slow
under this system. Finally, C. tuberosis undergoes automixis with gamete duplication [
43
], in which the
haploid egg cell divides and the cleavage nuclei fuse to restore diploidy or the egg cell fails to divide
following chromosome replication. This leads to individuals that are completely homozygous within a
single generation.
Given that female reproductives in AQS species can produce both sexual and asexual offspring,
how do they control which eggs are fertilized and which eggs are not? In the Hymenoptera, fertilized
eggs develop into females and unfertilized eggs develop into males [
48
], and queens can control
whether sperm is released by the spermatheca as the eggs are laid [
49
]. However, termites lack the
ability to regulate release of sperm during oviposition. Yashiro and Matsuura [
50
] showed that eggs
laid by the termite R. speratus differ in the number of micropyles, the pores through which sperm
can penetrate the egg. These authors found that some eggs lacked any micropyles and these eggs
developed parthenogenetically. The micropyle number changes with the age of queens, with older
queens producing fewer micropyles and a higher frequency of eggs lacking micropyles. Thus, queens
appear to produce asexual offspring under certain physiological conditions where they lay eggs lacking
micropyles. The physiological mechanisms regulating the formation of micropyles and the molecular
pathways underlying these mechanisms are yet to be determined.
Under AQS, parthenogenetically produced daughters are biased to develop into neotenics.
How did these two phenomena become linked? Nozaki et al. [
51
] provide evidence that termites may
have a preadaptation to the linkage between parthenogenetic reproduction and neotenic development.
These authors set up colonies of the non AQS species R. okinawanus headed by two female alates.
Such female–female pairs laid eggs, and although their eggs had a much lower hatch rate than
those of R. speratus, an AQS species, the successfully produced parthenogens were homozygous and
biased to develop into neotenics; one-third of parthenogens developed into neotenics compared to
none of the sexually produced females. Apparently, homozygosity, a result of parthenogenesis via
automixis with terminal fusion, somehow canalizes females to develop into neotenics, and this effect
of parthenogenesis on developmental fate predates the evolution of AQS.
Insects 2019,10, 52 8 of 12
In a surprising discovery, Yashiro et al. [
52
] found populations of all female colonies in the
drywood termite Glyptotermes nakajimai. This species occurs along coastal regions and nearby islands
in southern Japan. The authors found that 6 of 10 surveyed populations were comprised of asexual
colonies (n = 2–15 colonies sampled), whereas four populations had only sexual colonies. In asexual
colonies, all workers and soldiers were females, there were no kings and queens lacked sperm in
the spermatheca. There were often multiple primary queens in these colonies, up to 16, and many
had from 1 to 5 neotenic queens. In contrast, sexual colonies had both male and female workers and
soldiers; kings were present and queens were inseminated. Interestingly, most colonies of the sexual
lineage also had multiple primary queens and often multiple primary kings, and many had multiple
secondary kings and queens. The authors conducted a phylogenetic analysis and concluded that the
asexual lineage arose a single time from the sexual lineage some 14 million years ago. The asexual
colonies had fewer soldiers, but the soldiers were more uniform in their headwidth, leading the
authors to conclude that the soldiers in the asexual lineage were more efficient in their defensive
role. Primary reproductives of the sexual lineage are capable of producing some viable asexual eggs,
but these hatched at a much lower frequency than sexual eggs from this lineage or the asexual eggs of
the asexual lineage.
Yashiro et al. [
52
] hypothesized that there were a number of traits in the ancestors of the asexual
lineage of G. nakajimai that pre-adapted it to the evolution of all female societies. The first is the
occurrence of tachyparthenogenesis, the development of some unfertilized eggs laid by normally
sexually producing females. The second trait is pleometrosis, cooperative colony founding by queens,
which appears to be common in the sexual lineage. By grouping together, termite queens can survive
colony foundation through mutual grooming to avoid disease infection. Finally, the authors suggest
that being a single piece nester, as is true of all drywood termites and many other termites, individuals
do not have to leave the nest to forage for food, reducing the risk of exposure to many environmental
factors, such as pathogens, parasites and other natural enemies, in which genetic diversity may be
advantageous. Without the biotic pressure to reshuffle the genome through sexual reproduction to
better deal with natural enemies, selection may have favored all female colonies, resulting in higher
fitness of the female reproductives. The discovery of an asexual lineage in one termite species certainly
opens the door to finding additional cases in other termites.
3.5. Ecological and Evolutionary Factors Shaping Termite Breeding Systems
The large variation in colony breeding structure within and among species [
2
] raises the
question of the selective forces favoring one breeding system over another, for example, when are
extended families favored over simple families, or when are mixed families favored over extended
families?
Vargo et al.
[
24
] studied several populations of R. flavipes, along its range in the eastern U.S.,
and R. grassei, in its range in the Iberian Peninsula and southwestern France. In R. flavipes, these
authors found a negative relationship between latitude and both the proportion of colonies that are
simple families and the level of inbreeding in colonies. That is to say, populations in the southern
portions of its range had mainly simple family colonies and those in the northern portion had mainly
extended families (see Figure 1). Correspondingly, populations in the southern part of the range were
less inbred than those in more northern locations. R. grassei showed a similar relationship, but only the
increase in inbreeding with latitude was significant. Colonies that produce neotenics can replace their
primary reproductives when they die. Thus, differences among colonies in their breeding system are
largely due to age. Clearly, colonies in the southern portion of the ranges of these species either do
not live long enough to produce neotenics, or they do not live long after neotenics develop within the
colony, possibly because the inbred colonies are disadvantaged compared to the outbred colonies.
To investigate the possible role of local environmental conditions in shaping colony breeding
structure, Vargo et al. [
24
] examined several bioclimatic factors in relation to colony breeding structure
and the level of inbreeding in R. flavipes and R. grassei. In both species, the degree of inbreeding was
higher in cooler, moister habitats. In R. flavipes, inbreeding was most strongly affected by climatic
Insects 2019,10, 52 9 of 12
conditions (mean annual temperature and seasonality), whereas in R. grassei, biological variables,
such as the availability of wood resources and soil composition, were most closely associated with
inbreeding. These results suggest that local ecological conditions may be important in influencing
colony breeding structure, but how they favor one breeding system over another is not understood.
In the higher termite Macrotermes michaelseni, Brandl et al. [
36
] found geographic variation in the
frequency of polygyny (multiple queen colonies); colonies at the edge of its range were more likely to
be polygynous. The authors suggest that it is more difficult for monogamous pairs of reproductives
to found colonies near the border of their range due to ecological constraints, and groups of female
primary reproductives have a better chance of founding colonies successfully under such conditions.
Clearly, a more detailed understanding of the ecological factors influencing colony breeding structure
is an important area for future studies.
4. Conclusions
There is tremendous diversity in the breeding system of termites both within and among species.
In addition, the breeding system within a colony may change as the colony ages; most often, this
involves a colony transitioning from a simple family to an extended family following the death of
one or both of the primary reproductives and the production of replacement neotenics. It can also
occur when two or more colonies fuse to form a mixed family colony. Thanks to molecular genetic
techniques, there is an increasing number of studies on the breeding systems of termites yielding more
detailed variation about how colonies within a population differ and how populations within a species
can vary. One of the most exciting new developments is the discovery that several termite species,
including both lower and higher termites, use parthenogenetic reproduction as an integral part of
their lifecycle. In the coming years, there are sure to be many more species found that use asexual
reproduction. Continuing to document the range of breeding systems in termites using a combination
of field collections and genetic studies will remain an important area of research.
Although our understanding of the variation in breeding systems has been rapidly increasing,
our knowledge of the evolutionary and ecological factors responsible for this variation lags far
behind. Studies are needed to investigate the relationship between breeding system and ecological
conditions, such as temperature, availability of food resources, population density, pathogen pressure,
etc., as well as life history attributes such as nesting habits, food resources utilized (wood, soil,
fungus) and phylogenetic relationships. In addition, laboratory and field studies investigating the
possible deleterious effects of inbreeding are also needed to better understand the costs and benefits of
inbreeding in extended family colonies or the lack of inbreeding under AQS. Variation in breeding
systems is common throughout social insects. Termites provide an excellent group for providing
insights into the forces shaping breeding systems in social insects.
Funding: This research received no external funding.
Conflicts of Interest: The author declares no conflict of interest.
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