Divergence in style length and pollen size leads to a postmating-prezygotic reproductive barrier among populations of Silene latifolia.

A central tenet of speciation research is the need to identify reproductive isolating barriers. One approach to this line of research is to identify the phenotypes that lead to reproductive isolation. Several studies on flowering plants have shown that differences in style length contribute to reproductive isolation between species, leading us to consider whether style length could act as a reproductive barrier among populations of a single species. This could occur if style length varied sufficiently and pollen size covaried with style length. Populations of Silene latifolia exhibit variation in flower size, including style length, that is negatively correlated with annual precipitation. We show that this divergence in style length has a genetic basis and acts as a reproductive barrier: males from small-flowered populations produced relatively small pollen grains that were poor at fertilizing ovules when crossed to females from large-flowered populations, leading to a significant reduction in seed production. Manipulating the distance pollen tubes had to travel revealed that this failure was purely mechanical and not the result of other incompatibilities. These results show that style length acts as a postmating-prezygotic reproductive barrier and indicate a potential link between ecotypic differentiation and reproductive isolation within a species.

Differences in style length confer prezygotic isolation between two dioecious species of Silene in sympatry.

One fundamental signature of reinforcement is elevated prezygotic reproductive isolation between related species in sympatry relative to allopatry. However, this alone is inadequate evidence for reinforcement, as traits conferring reproductive isolation can occur as a by-product of other forces. We conducted crosses between Silene latifolia and S. diclinis, two closely related dioecious flowering plant species. Crosses with S. latifolia mothers from sympatry exhibited lower seed set than mothers from five allopatric populations when S. diclinis was the father. However, two other allopatric populations also exhibited low seed set. A significant interaction between style length and sire species revealed that seed set declined as style length increased when interspecific, but not intraspecific, fathers where used. Moreover, by varying the distance pollen tubes had to traverse, we found interspecific pollen placement close to the ovary resulted in seed set in both long- and short-styled S. latifolia mothers. Our results reveal that the long styles of S. latifolia in sympatry with S. diclinis contribute to the prevention of hybrid formation. We argue that forces other than reinforcing selection are likely to be responsible for the differences in style length seen in sympatry.

Genetic architecture of floral traits in Iris hexagona and Iris fulva.

The formation of hybrids among closely related species has been observed in numerous plant taxa. Selection by pollinators on floral traits can act as an early reproductive isolating barrier and may be especially important when there is overlap in distribution and flowering time. In this study, we use Quantitative Trait Locus (QTL) mapping based on 293 codominant SNP markers in an F2 population (n = 328) to assess the size, magnitude, and location of the genetic regions controlling floral traits known to be important for pollinator attraction in 2 species of Lousiana Irises, Iris fulva and Iris hexagona. We also evaluate correlations among F2 traits and identify transgression in the hybrid population. Overall, we observe that differences in most floral traits between I. fulva and I. hexagona are controlled by multiple QTLs and are distributed across several linkage groups. We also find evidence of transgression at several QTL, suggesting that hybridization can contribute to generating phenotypic variation, which may be adaptive in rapidly changing environments.

QTL mapping reveals the genetic architecture of loci affecting pre- and post-zygotic isolating barriers in Louisiana Iris.

BACKGROUND: Hybridization among Louisiana Irises has been well established and the genetic architecture of reproductive isolation is known to affect the potential for and the directionality of introgression between taxa. Here we use co-dominant markers to identify regions where QTL are located both within and between backcross maps to compare the genetic architecture of reproductive isolation and fitness traits across treatments and years. RESULTS: QTL mapping was used to elucidate the genetic architecture of reproductive isolation between Iris fulva and Iris brevicaulis. Homologous co-dominant EST-SSR markers scored in two backcross populations between I. fulva and I. brevicaulis were used to generate genetic linkage maps. These were used as the framework for mapping QTL associated with variation in 11 phenotypic traits likely responsible for reproductive isolation and fitness. QTL were dispersed throughout the genome, with the exception of one region of a single linkage group (LG) where QTL for flowering time, sterility, and fruit production clustered. In most cases, homologous QTL were not identified in both backcross populations, however, homologous QTL for flowering time, number of growth points per rhizome, number of nodes per inflorescence, and number of flowers per node were identified on several linkage groups. CONCLUSIONS: Two different traits affecting reproductive isolation, flowering time and sterility, exhibit different genetic architectures, with numerous QTL across the Iris genome controlling flowering time and fewer, less distributed QTL affecting sterility. QTL for traits affecting fitness are largely distributed across the genome with occasional overlap, especially on LG 4, where several QTL increasing fitness and decreasing sterility cluster. Given the distribution and effect direction of QTL affecting reproductive isolation and fitness, we have predicted genomic regions where introgression may be more likely to occur (those regions associated with an increase in fitness and unlinked to loci controlling reproductive isolation) and those that are less likely to exhibit introgression (those regions linked to traits decreasing fitness and reproductive isolation).

Haldane's rule is extended to plants with sex chromosomes.

Haldane's rule is an empirical phenomenon that has been observed in animals with sex chromosomes. The rule states that the heterogametic sex (XY or ZW) will be "absent, rare, or sterile" following hybridization between two species. Despite the near ubiquity of Haldane's rule in animal hybridizations, it has not been documented in organisms other than animals. Here, we show evidence for both rarity and sterility in hybrid male but not female offspring in crosses between three dioecious plant species from the genus Silene with heteromorphic (XY) sex chromosomes. Our results are consistent with Haldane's rule, extending its applicability to plants with sex chromosomes.