[Show abstract][Hide abstract] ABSTRACT: Reciprocal crosses between species often display an asymmetry in the fitness of F1 hybrids. This pattern, referred to as isolation asymmetry or Darwin's corollary to Haldane's rule, is a general feature of reproductive isolation in plants, yet factors determining its magnitude and direction remain unclear. We evaluated reciprocal species crosses between two naturally hybridizing diploid species of Arabidopsis to assess the degree of isolation asymmetry at different postmating life stages. We found that pollen from Arabidopsis arenosa will usually fertilize ovules from Arabidopsis lyrata; the reverse receptivity being less complete. Maternal A. lyrata parents set more F1 hybrid seed, but germinate at lower frequency, reversing the asymmetry. As predicted by theory, A. lyrata (the maternal parent with lower seed viability in crosses) exhibited accelerated chloroplast evolution, indicating that cytonuclear incompatibilities may play a role in reproductive isolation. However, this direction of asymmetrical reproductive isolation is not replicated in natural suture zones, where delayed hybrid breakdown of fertility at later developmental stages, or later-acting selection against A. arenosa maternal hybrids (unrelated to hybrid fertility, e.g., substrate adaptation) may be responsible for an excess of A. lyrata maternal hybrids. Exogenous selection rather than cytonuclear incompatibilities thus shapes the asymmetrical postmating isolation in nature.
Full-text · Article · Mar 2015 · Ecology and Evolution
[Show abstract][Hide abstract] ABSTRACT: The hybrid zone on Mount Etna (Sicily) between Senecio aethnensis and Senecio chrysanthemifolius (two morphologically and physiologically distinct species) is a classic example of an altitudinal cline. Hybridization at intermediate altitudes and gradients in phenotypic and life-history traits occur along altitudinal transects of the volcano. The cline is considered to be a good example of ecological selection with species differences arising by divergent selection opposing gene flow. However, the possibility that the cline formed from recent secondary contact following an allopatric phase is difficult to exclude. We demonstrate a recent split between S. aethnensis and S. chrysanthemifolius (as recent as ∼32,000 years ago) and sufficient gene flow (2Nm > 1) to have prevented divergence (implicating a role for diversifying selection in the maintenance of the cline). Differentially expressed genes between S. aethnensis and S. chrysanthemifolius exhibit significantly higher genetic divergence relative to "expression invariant" controls, suggesting that species differences may in part be mediated by divergent selection on differentially expressed genes involved with altitude-related adaptation. The recent split time and the absence of fixed differences between these two ecologically distinct species suggest the rapid evolution to an altitudinal cline involving selection on both sequence and expression variation.
[Show abstract][Hide abstract] ABSTRACT: The mechanics of speciation with gene flow are still unclear. Disparity among genes in population differentiation (F(ST)) between diverging species is often interpreted as evidence for semipermeable species boundaries, with selection preventing "key" genes from introgressing despite ongoing gene flow. However, F(ST) can remain high before it reaches equilibrium between the lineage sorting of species divergence and the homogenizing effects of gene flow (via secondary contact). Thus, when interpreting F(ST), the dynamics of drift, gene flow, and selection need to be taken into account. We illustrate this view with a multigenic analyses of gene flow and selection in three closely related Silene species, S. latifolia, S. dioica, and S. diclinis. We report that although S. diclinis appears to have evolved in allopatry, isolation with (bidirectional) gene flow between S. latifolia and S. dioica is likely, perhaps as a result of parapatric speciation followed by more extensive sympatry. Interestingly, we detected the signatures of apparently independent instances of positive selection at the same locus in S. latifolia and S. dioica. Despite gene flow between the species, the adaptive alleles have not crossed the species boundary, suggesting that this gene has independently undergone species-specific (diversifying or parallel) selection.
[Show abstract][Hide abstract] ABSTRACT: Natural selection can reduce the effective population size of the nonrecombining Y chromosome, whereas local adaptation of Y-linked genes can increase the population divergence and overall intra-species polymorphism of Y-linked sequences. The plant Silene latifolia evolved a Y chromosome relatively recently, and most known X-linked genes have functional Y homologues, making the species interesting for comparisons of X- and Y-linked diversity and subdivision. Y-linked genes show higher population differentiation, compared to X-linked genes, and this might be maintained by local adaptation in Y-linked genes (or low sequence diversity). Here we attempt to test between these causes by investigating DNA polymorphism and population differentiation using a larger set of Y-linked and X-linked S. latifolia genes (than used previously), and show that net sequence divergence for Y-linked sequences (measured by D(a) , also known as δ) is low, and not consistently higher than X-linked genes. This does not support local adaptation, instead, the higher values of differentiation measures for the Y-linked genes probably result largely from reduced total variation on the Y chromosome, which in turn reflect deterministic processes lowering effective population sizes of evolving Y-chromosomes.
[Show abstract][Hide abstract] ABSTRACT: Gene flow occurs predominantly via pollen in angiosperms, leading to stronger population subdivision for maternally inherited markers, relative to paternally or biparentally inherited genes. In contrast to this trend, population subdivision within Silene latifolia and S. dioica, as well as subdivision between the two species, is substantially lower in maternally inherited chloroplast genes compared to paternally inherited Y-linked genes. A significant frequency spectrum bias toward rare polymorphisms and a significant loss of polymorphism in chloroplast genes compared to Y-linked and autosomal genes suggest that intra- and inter-specific subdivision in the chloroplast DNA may have been eroded by a selective sweep that has crossed the S. latifolia and S. dioica species boundary.
[Show abstract][Hide abstract] ABSTRACT: Quercus petraea and Quercus robur are two closely related oak species, considered to hybridize. Genetic markers, however, indicate that despite sharing most alleles, the two species remain separate genetic units. Analysis of 20 microsatellite loci in multiple populations from both species suggested a genome-wide differentiation. Thus, the allele sharing between both species could be explained either by low rates of gene flow or shared ancestral variation. We performed further analyses of population differentiation in a biogeographical setting and an admixture analysis in mixed oak stands to distinguish between both hypotheses. Based on our results we propose that the low genetic differentiation among these species results from shared ancestry rather than high rates of gene flow.
[Show abstract][Hide abstract] ABSTRACT: Quercus petraea colonized Ireland after the last glaciation from refugia on mainland Europe. Deforestation, however, beginning in Neolithic times, has resulted in small, scattered forest fragments, now covering less than 12,000 ha.
Plastid (three fragments) and microsatellite variation (13 loci) were characterized in seven Irish populations sampled along a north-south gradient. Using Bayesian approaches and Wright's F-statistics, the effects of colonization and fragmentation on the genetic structure and mating patterns of extant oak populations were investigated.
All populations possessed cytotypes common to the Iberian Peninsula. Despite the distance from the refugial core and the extensive deforestation in Ireland, nuclear genetic variation was high and comparable to mainland Europe. Low population differentiation was observed within Ireland and populations showed no evidence for isolation by distance. As expected of a marker with an effective population size of one-quarter relative to the nuclear genome, plastid variation indicated higher differentiation. Individual inbreeding coefficients indicated high levels of outcrossing.
Consistent with a large effective population size in the historical migrant gene pool and/or with high gene flow among populations, high within-population diversity and low population differentiation was observed within Ireland. It is proposed that native Q. petraea populations in Ireland share a common phylogeographic history and that the present genetic structure does not reflect founder effects.
[Show abstract][Hide abstract] ABSTRACT: Quercus petraea and Quercus robur are two closely related oak species that frequently hybridize. We sequenced 70 clones containing the 5.8S and ITS2 regions of ribosomal DNA (rDNA) from these two species and did not detect a species-specific difference. Surprisingly, three divergent (up to 12.6%) rDNA families were identified in both species, indicating that they predate the speciation event. Despite a large between-rDNA-families divergence, rDNA sequences were very similar within families, suggesting ongoing concerted evolution. Expression analysis, relative-rate tests, and mutation spectrum analyses indicated that only a single rDNA family is functional. We propose that past hybridization events, combined with nucleolar dominance, were the evolutionary processes underlying the contemporary rDNA variability in Q. petraea and Q. robur.
Full-text · Article · Mar 2001 · Molecular Biology and Evolution
[Show abstract][Hide abstract] ABSTRACT: The two widespread species of oak tree in Europe, Quercus robur L. and Q. petraea (Matt.) Liebl., hybridize extensively, calling their taxonomic status into question. Here we use microsatellite DNA, a highly informative genetic marker, to show that Q. robur and Q. petraea are discrete taxonomic units despite this intensive hybridization. Furthermore, individual oaks can be assigned to separate species.