Genetic architecture of conspecific sperm precedence in Allonemobius fasciatus and A. socius.

The evolution of barriers to gene exchange is centrally important to speciation. We used the crickets Allonemobius fasciatus and A. socius to investigate the genetic architecture of conspecific sperm precedence (CSP), a postinsemination prezygotic reproductive barrier. With amplified fragment-length polymorphism (AFLP) markers and controlled crosses we constructed linkage maps and estimated positions of QTL associated with CSP. The majority of QTL have low to moderate effects, although a few QTL exist in A. socius with large effects, and the numbers of QTL are comparable to numbers of genes accounting for species differences in other studies. The QTL are spread across many unlinked markers, yet QTL placed with linked markers are on a small number of linkage groups that could reflect the role of the large Allonemobius sex chromosome in prezygotic isolation. Although many QTL had positive effects on conspecific sperm utilization several QTL also exerted negative effects, which could be explained by intraspecific sexual conflict, sperm competition, or epistasis of introgressed genes on novel backgrounds. One unexpected outcome was that A. socius CSP alleles have a stronger effect than those from A. fasciatus in hybrid females, causing hybrids to behave like A. socius with regard to sperm utilization. Implications of this asymmetry in the Allonemobius hybrid zone are discussed.

The genetics of reproductive isolation: a retrospective and prospective look with comments on ground crickets.

An intriguing aspect of the current renaissance in investigations of the genetics of reproductive isolation is that it has been dominated by studies that resemble work done in the 1930s, 1940s, and 1950s. The dominant model organism (Drosophila), research approaches, and traits of interest (sterility and inviability of hybrids) all harken back to this earlier era. Herein, we explore the factors that led to a rebirth of interest in the genetics of reproductive isolation and to the adoption of the approaches of an earlier generation of biologists. At the same time, we appeal for more intensive investigations of traits that reproductively isolate closely related species, inclusion of a greater range of organisms in studies of reproductive isolation, and focus on a broader range of questions surrounding speciation. We end with a description of ongoing quantitative trait loci (QTL) studies of conspecific sperm precedence in the ground crickets Allonemobius fasciatus and Allonemobius socius. We have found several QTL with large effects on variance in patterns of sperm utilization in backcross females. Moreover, some QTL have an antagonistic effect on conspecific sperm, a finding that lends support to the hypothesis that rapid evolution of conspecific sperm precedence is a by-product of sexual conflict.

CONSPECIFIC SPERM PRECEDENCE IS AN EFFECTIVE BARRIER TO HYBRIDIZATION BETWEEN CLOSELY RELATED SPECIES.

Conspecific sperm precedence is widespread in animals, appears to evolve rapidly, and is thought to have the potential to prevent hybridization between closely related species. However, to date no study has tested the isolating potential of such a barrier in mixed populations of two taxa under conditions in which other potential barriers to gene flow are controlled for or are prevented from operating. We tested the isolating potential of conspecific sperm precedence in the ground crickets Allonemobius fasciatus and A. socius in population cage experiments in which the frequency of the two species was varied. Despite the observation of abundant interspecific matings, the proportions of hybrid progeny were low and differed statistically from the proportions expected in the absence of conspecific sperm precedence. The results demonstrate that conspecific sperm precedence can severely limit gene flow between closely related species, even when one species is less abundant than the other.