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Parthenogenesis: Birth of a New Lineage or
Casper J. van der Kooi and Tanja Schwander
University of Lausanne, Department of Ecology and Evolution, Le Biophore, CH – 1015 Lausanne, Switzerland
Correspondence: Casper.vanderKooi@unil.ch (C.J.v.d.K.), Tanja.Schwander@unil.ch (T.S.)
Parthenogenesis — the ability to produce offspring from unfertilized eggs — is widespread among
invertebrates and now increasingly found in normally sexual vertebrates. Are these cases reproductive
errors or could they be a ﬁrst step in the emergence of new parthenogenetic lineages?
The phenomenon of virgin birth has long
fascinated scientists and laymen alike.
The ﬁrst account of parthenogenesis in
the literature is the prophecy of Jesus
Christ’s birth in Isaiah 7:14: ‘‘Therefore the
Lord himself will give you a sign: The virgin
will conceive and give birth to a son, and
will call him Immanuel’’. This reference to
parthenogenesis is unusual in two ways:
ﬁrst, it is the only account of ‘natural
parthenogenesis’ in a mammal. Mammals
are believed to be completely unable to
reproduce via parthenogenesis because
of a number of developmental and genetic
constraints . Second, while the
‘‘Blessed Virgin Mary’’ might have been
able to conceive a daughter via
parthenogenesis, the conception of a
son is highly unlikely. As male sex in
humans is determined by genes on the
Y chromosome, Mary, as a woman,
could not have transmitted any
Y chromosomes to her offspring. In
contrast to humans, parthenogenetic
production of sons is expected in species
with other types of sex determination.
For example, in birds, some reptiles and
Current Biology 25, R654–R676, August 3, 2015 ª2015 Elsevier Ltd All rights reserved R659
butterﬂies, females are the heterogametic
sex: they carry two differentiated sex
chromosomes (Z and W) while males
are homogametic (ZZ). This sex
determination system can result in the
virgin birth of sons if parthenogenesis is
automictic , whereby oocytes undergo
a normal meiosis that results in four
haploid products (one egg cell and three
polar bodies). Under sexual reproduction,
the polar bodies would degenerate and
the egg cell would fuse with a sperm
to generate a diploid zygote. Under
automictic parthenogenesis, diploidy is
restored via the fusion of the egg cell
and one of the polar bodies .As
in self-fertilization or mating with
relatives, automictic parthenogenesis
can cause an increase in homozygosity,
including homozygosity at the sex
chromosome. Hence ZW females can
parthenogenetically produce ZZ sons
and ZW daughters (WW individuals are
generally not viable). Thus, automictic
parthenogenesis is the mechanism
underlying the occasional production
of sons and daughters well known for
many species of bagworm moths 
and recently described in some reptile
species, including Komodo dragons 
and snakes .
While male-producing parthenogenesis
is rare, female-producingparthenogenesis
is widespread among animals and
mostly obligate (Figure 1), with many
documented cases in species-rich
invertebrate groups such as insects,
nematodes and crustaceans, and with
only few examples in vertebrates .A
recent paper by Fields et al.  in Current
Biology documents a new case of
female-producing parthenogenesis in
a critically endangered ray species,
the sawﬁsh Pristis pectinata.Pristis
species — like all sawﬁsh — are
characterized by their elongated ﬂat nose
lined with teeth. P. pectinata occurs in
coastal areas of the western Atlantic sea,
mostly in bay areas around Florida. A
microsatellite-based genetic screen of
190 individuals in a wild population
revealed seven females that were most
likely produced via parthenogenesis.
Parthenogenetic ancestry was deduced
because the seven females were
homozygous at all or almost all screened
loci . Mating between relatives can also
result in homozygosity, but individuals in
the screened population were not related
to each other. Furthermore, mating
between close relatives is unlikely given
the ecology of the species, leaving
parthenogenesis as the most likely
Are these parthenogenetically
produced sawﬁsh females rare
‘accidents’, or could they be indicative
of a unique case of adaptive, facultative
parthenogenesis in a vertebrate?
Facultative parthenogenesis, where
an individual female can produce
offspring either sexually or
parthenogenetically (Figure 1), is
exceedingly rare among animals. A
much more widespread phenomenon
is accidental parthenogenesis — also
called spontaneous parthenogenesis,
or tychoparthenogenesis : the hatching
of a very small proportion of unfertilized
eggs in a normal, sexual species.
For example, in different species of
Drosophila, observed rates of accidental
parthenogenesis in natural populations
range from one egg in 100,000 to one in a
million successfully developing into an
adult . These hatching rates are orders
of magnitude lower than for facultative
parthenogenesis, where the majority of
unfertilized eggs hatch. However, without
systematic screens for hatching success
of eggs laid by virgin females, accidental
and facultative parthenogenesis can be
difﬁcult to distinguish. Widely popularized
examples of rare parthenogenesis in
vertebrates are typically interpreted as
facultative parthenogenesis , including
reports of parthenogenesis in sharks
[10,11], snakes  and Komodo dragons
, producing offspring while kept
solitarily in captivity. Given the current
evidence, these examples are, however,
most likely cases of accidental rather than
facultative parthenogenesis, mimicking
the high incidence of accidental
parthenogenesis among invertebrates.
Distinguishing whether the sawﬁsh
females are a case of facultative,
accidental, or obligate parthenogenesis
would require additional studies, ideally
involving breeding experiments. Although
such experiments might be difﬁcult to
conduct with P. pectinata because of
its ecological requirements, they could
generate interesting insights into the
evolution of parthenogenesis. For
example, because of its great inefﬁciency,
accidental parthenogenesis [7,8] is
generally not adaptive (Figure 1). An
exception might be situations where
sexual females fail to ﬁnd a mate. Stalker
 predicted that in marginal populations
or other situations where mates are
limited even inefﬁcient accidental
parthenogenesis could be adaptive and
thus selectively favored. This prediction
is supported by evidence from natural
populations of Drosophila vinegar ﬂies
and stick insects. In these species,
accidental parthenogenesis rates are
especially high in low-density populations
where large fractions of adult females
remain unmated [13,14]. Thus, via the
accumulation of gradual changes,
accidental parthenogenesis might be a
stepping-stone to ‘true’ parthenogenesis
and give rise to new facultative or obligate
Female may reproduce via
sex and/or parthenogenesis
Figure 1. The efﬁciency of parthenogenesis varies widely.
Accidental parthenogenesis refers to the very rare hatching of unfertilized eggs in sexual populations,
often due to reproductive errors, that can generate male offspring in species with female heterogame ty.
Given the very low hatching success, accidental parthenogenesis is often not adaptive. Under
facultative parthenogenesis a female may reproduce via sex and/or parthenogenesis; hence this
reproductive mode combines the advantages of sex and parthenogene sis. Under obligate
parthenogenesis, females cannot reproduce sexually at all, even if mated to males of sexual lineages.
Populations consisting solely of obligate parthenogens are characterized by the virtual absence of
males. However, many species feature mixed reproduction, with some females reproducing sexually
and others via obligate parthenogenesis. These species are characterized by sex ratios ranging from
50:50 to strongly female-biased.
R660 Current Biology 25, R654–R676, August 3, 2015 ª2015 Elsevier Ltd All rights reserved
So, could accidental parthenogenesis
in humans ever give rise to a new
parthenogenetic lineage? Probably
not, as the developmental and genetic
constraints in humans and other
mammals would most likely prevent the
emergence of adaptive parthenogenesis
in natural populations . As it turns out,
even the most famous speculation about
parthenogenesis, Jesus Christ’s birth,
owes its existence not to a miracle but to a
human error during the translation of
Isaiah 7:14 from Hebrew to Greek: The
Hebrew word almah can refer to a young
woman of marriageable age, whether
married or not . The ‘young woman’
became a ‘virgin’ in the gospel according
to Matthew, where almah was translated
as the Greek parthenos.
1. Engelstaedter, J. (2008). Constraints on the
evolution of asexual reproduction. BioEssays
2. Suomalainen, E., Saura, A., and Lokki, J.
(1987). Cytology and Evolution in
Parthenogenesis (Boca Raton: CRC Press).
3. Seiler, J. (1960). Untersuchungen u
Entstehung der Parthenogenese bei Solenobia
triquetrella F.R. (Lepidoptera, Psychidae) II.
Analyse der diploid parthenogenetischen
S. triquetrella. Verhalten, Aufzuchtresultate
und Zytologie. Chromosoma 11, 29–102.
4. Watts, P.C., Buley, K.R., Sanderson, S.,
Boardman, W., Cioﬁ, C., and Gibson, R.
(2006). Parthenogenesis in Komodo dragons.
Nature 444, 1021–1022.
5. Booth, W., Johnson, D.H., Moore, S., Schal,
C., and Vargo, E.L. (2011). Evidence for viable,
non-clonal but fatherless Boa constrictors.
Biol. Lett. 7, 253–256.
6. Fields, A.T., Feldheim, K.A., Poulakis, G.R.,
and Chapman, D.D. (2015). Facultative
parthenogenesis in a critically endangered
wild vertebrate. Curr. Biol. 25, R446–R447.
7. Bell, G. (1982). The Masterpiece of Nature: The
Evolution and Genetics of Sexuality (Berkeley,
CA: University of California Press).
8. Templeton, A.R. (1979). The parthenogenetic
capacities and genetic structures of sympatric
populations of Drosophila mercatorum and
Drosophila hydei. Genetics 92, 1283–1293.
9. Lampert,K. (2008). Facultativeparthenogenesis
in vertebrates: reproductive error or chance?
Sex. Dev. 2, 290–301.
10. Chapman, D.D., Shivji, M.S., Louis, E.,
Sommer, J., Fletcher, H., and Prodo
(2007). Virgin birth in a hammerhead shark.
Biol. Lett. 3, 425–427.
11. Feldheim, K.A., Chapman, D.D., Sweet, D.,
Fitzpatrick, S., Prodo
¨hl, P.A., Shivji, M.S., and
Snowden, B. (2010). Shark virgin birth
produces multiple, viable offspring. J. Hered
12. Stalker, H.D. (1956). On the evolution of
parthenogenesis in Lonchoptera (Diptera).
13. Kramer, M.G., and Templeton, A.R. (2001).
Life-history changes that accompany the
transition from sexual to parthenogenetic
reproduction in Drosophila mercatorum.
Evolution 55, 748–761.
14. Schwander, T., Vuilleumier, S., Dubman, J.,
and Crespi, B.J. (2010). Positive feedback in
the transition from sexual reproduction to
parthenogenesis. Proc. R. Soc. B. Biol. Sci.,
15. Argyle, A.W. (1963). The gospel according to
Matthew, Volume 33 (Cambridge: Cambridge
Binocular Vision: Joining Up the Eyes
Andrew T. Smith
Department of Psychology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK
To provide a unitary view of the external world, signals from the two eyes must be combined: a new study
pinpoints the location in the human brain where the requisite combination occurs.
A fundamental feature of human vision is
that, despite having two eyes, we
normally see only one representation of
the world around us. This phenomenon,
imaginatively termed cyclopean
perception by the late Bela Julesz ,
requires a seamless combination of two
completely separate neural signals and
imposes on the brain a substantial
computational burden that a cyclops
would be spared. There are, however,
a number of beneﬁts to having two
eyes that collectively outweigh the
computational cost. Perhaps the most
obvious, although not necessarily the
evolutionary driver, is insurance against
loss of an eye. Another is that it permits a
wider ﬁeld of view (only modestly wider in
humans but much wider in horses, sheep
and many other mammals). The most
studied beneﬁt is that having two eyes
permits stereoscopic vision: the
construction of accurate estimates of the
distances of nearby objects based on
subtle differences between the two retinal
images. These beneﬁts depend on the
replacement of two representations of the
world by a single, cyclopean
representation. Where in the brain does
this happen? It might be expected that a
harmonious coalition of left and right
would be constructed at the very ﬁrst
processing stage at which both signals
are present in proximity: the thalamus;
however, it has long been known that this
is not the case and that the answer is
‘‘somewhere in the visual cortex’’. In this
issue of Current Biology, Barendregt et al.
 present evidence from functional
magnetic resonance imaging (fMRI) that
the transformation occurs between the
primary visual cortex, known as V1, and
the second visual area, V2.
Whether a given neuron is responsive to
light stimulation in either eye or is driven
only by one eye has been addressed in
many neurophysiological studies, starting
with the pioneering work of Nobel Prize
winners Hubel and Wiesel, who found that
the primary visual cortex of macaques
contains a mixed bag of cells, some
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