Academic journal article Genetics

The Genetic Basis of Prezygotic Reproductive Isolation between Drosophila Santomea and D. Yakuba Due to Mating Preference

Academic journal article Genetics

The Genetic Basis of Prezygotic Reproductive Isolation between Drosophila Santomea and D. Yakuba Due to Mating Preference

Article excerpt

ABSTRACT

Sexual isolating mechanisms that act before fertilization are often considered the most important genetic barriers leading to speciation in animals. While progress has been made toward understanding the genetic basis of the postzygotic isolating mechanisms of hybrid sterility and inviability, little is known about the genetic basis of prezygotic sexual isolation. Here, we map quantitative trait loci (QTL) contributing to prezygotic reproductive isolation between the sibling species Drosophila santomea and D. yakuba. We mapped at least three QTL affecting discrimination of D. santomea females against D. yakuba males: one X-linked and one autosomal QTL affected the likelihood of copulation, and a second X chromosome QTL affected copulation latency. Three autosomal QTL also affected mating success of D. yakuba males with D. santomea. No epistasis was detected between QTL affecting sexual isolation. The QTL do not overlap between males and females and are not disproportionately concentrated on the X chromosome. There was some overlap in map locations of QTL affecting sexual isolation between D. santomea and D. yakuba with QTL affecting sexual isolation between D. simulans and D. mauritiana and with QTL affecting differences in pigmentation between D. santomea and D. yakuba. Future high-resolution mapping and, ultimately, positional cloning, will reveal whether these traits do indeed have a common genetic basis.

DESPITE the probable importance of sexual isolation as a primary reproductive barrier during the process of speciation (COYNE and ORR 2004), we know relatively little about the genetic basis of interspecific mate discrimination. Yet such genetic studies can answer important questions about speciation. Is sexual isolation based on few genes or many? If many, do a few genes contribute to most of the sexual isolation? Do "mate discrimination" genes tend to occur in similar regions of chromosomes among different species in the same group, implying that sexual isolation may involve identical genes in different speciation events? Do the same chromosome regions (and possibly the same genes) contribute to mate preference in males and females? Finally, what is the normal function of genes involved in sexual isolation? This last question can be answered only by identifying those genes, an endeavor that must begin by their fine-structure localization.

Previous studies of prezygotic isolation in Drosophila have mapped genes affecting sexual isolation to whole chromosomes, chromosome arms, or large sections of chromosomes (ZOUROS 1981; COYNE 1989, 1993, 1996a,b; Wu et al 1995; NOOR 1997; TING et al 2001; WILLIAMS et al 2001; GLEASON and RITCHIE 2004; TAKAHASHI and TING 2004), but so far there have been only a few studies localizing quantitative trait loci (QTL) affecting sexual isolation between species with high resolution by linkage to molecular markers (CIVETTA and CANTOR 2003; MOEHRING et al. 2004)-the first step toward positional cloning of candidate loci. Here, we report the results of mapping QTL causing sexual isolation between two sister species, Drosophila yakuba and D. santomea, using 32 species-specific molecular markers to localize chromosome regions involved in mate discrimination. D. yakuba is widespread across sub-Saharan Africa and on islands near the continent, inhabiting open areas such as savannas, montane grassland, and, in human-colonized areas, disturbed habitats such as plantations and cut-over fields. D. santomea, discovered in 1998, is endemic to Sao Tome, an 860-km^sup 2^ volcanic island 255 km off the coast of Gabon (LACHAISE et al 2000), where it inhabits only montane rain and mist forests. D. yakuba also inhabits Sao Tome. On the mountain of Pico de Sao Tome, D. yakuba lives at elevations below 1450 m, while D. santomealives at elevations above 1150 m. Between these elevations, the species' ranges overlap at an ecotone between plantations and virgin rain forest, forming a hybrid zone in which one finds a low frequency (~1%) of hybrids (LACHAISE et al. …

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