Reproductive isolation in Caenorhabditis: terminal phenotypes of hybrid embryos.

SUMMARY Several interspecific combinations of the “ele-gans” group of Caenorhabditis species are cross-fertile. MostF1 hybrids from these crosses arrest during embryogenesis.Developmental defects observed in hybrid embryos includedefects in gastrulation initiation, defects in embryonic com-paction, and defects in embryonic elongation. These repro-ductive barriers have arisen multiple times in the evolution of Caenorhabditis. INTRODUCTION Reproductive isolation refers to all genetic mechanisms thatprevent or limit gene flow between populations (Mayr 1963;Dobzhansky 1970). These mechanisms are important as the ad-vent of reproductive isolation can result in speciation and/orenforce existing species boundaries. Relatively few genes in-volved in the implementation of reproductive isolation havebeen well-characterized (see Wu and Palopoli 1994). This isprimarily because the reproductive barriers erected by thesegenes usually preclude their characterization by traditional ge-netic methodologies. Thus, while a limited number of repro-ductively isolating genes in Drosophila have been well-mapped and well-characterized (e.g., Hutter and Ashburner1987; Hutter et al. 1990; Perez et al. 1993; Hutter and Karch1994; Perez and Wu 1995; Sawamura et al. 1995; Davis and Wu1996), most remain obscure. In species combinations in whichintrogression studies are not possible, three approaches areavailable for the identification of reproductive isolating genes.First, genes can be identified in variant strains that exhibit qual-itative differences in reproductive isolation (e.g., Watanabe1979; Davis et al. 1996); second, genes can be identified bycellular and biochemical studies of reproductive barriers (e.g.,Wittbrodt et al. 1989; Glabe and Clark 1991; Wittbrodt et al.1992); and third, candidate genes can be identified by compar-ing hybrid terminal phenotypes with nonhybrid mutant pheno-types (e.g., Orr et al. 1997). This last approach requires detailedstudy of development in hybrid animals.In the nematode genus of Caenorhabditis, mechanismsthat isolate four species, C. elegans , C. briggsae , C. remanei ,and C. sp. strain CB5161, have been described (Baird et al.1992). These species all are members of the “ elegans” groupof Caenorhabditis and are considered to be close relatives ofeach other (Sudhaus and Kiontke 1996). Among these fourspecies, no mate preferences have been observed. The phy-logeny of C. elegans, C. briggsae, C. remanei, and C. sp.strain CB5161, as inferred from 18s rDNA and RNA poly-merase II sequences, is shown in Fig. 1 (Baldwin et al. 1997).Reproductive isolating mechanisms that act among C. ele-gans, C. briggsae, C. remanei, and C. sp. strain CB5161 in-clude assortive fertilization, hybrid lethality, and hybrid ste-rility (Baird et al. 1992). Unfortunately, introgression studiesin these species combinations are not possible. Thus, theidentification of reproductive isolating genes in Caenorhab-ditis currently must rely upon studies of variant strains, oncellular and biochemical studies of reproductive barriers,and/or on comparisons of hybrid terminal phenotypes tononhybrid mutant phenotypes. Among the cross-fertile com-binations of Caenorhabditis are crosses of C. elegans , C. re-manei, and C. briggsae males to C. sp. strain CB5161 fe-males (Baird et al. 1992). Hybrids that result from thesecrosses arrest during embryogenesis. In this report, terminalphenotypes in hybrid embryos are described. It is hoped thatthese descriptions will facilitate the identification of genesthat implement these reproductive barriers.MATERIALS AND METHODS