HYBRIDIZATION FREQUENCY IS NEGATIVELY CORRELATED WITH DIVERGENCE TIME OF MITOCHONDRIAL DNA HAPLOTYPES IN A SEA STAR (LEPTASTERIAS SPP.) SPECIES COMPLEX.

Abstract

If interspecific reproductive isolation is caused by cumulative genetic change within species, then the degree of isolation of recently diverged species should increase with the amount of time since divergence from the common ancestral population (Coyne and Orr 1989). This prediction is fairly robust, being independent of whether or not control of reproductive isolation is polygenic, and excluding obvious exceptions like polyploidization. However, tests of this prediction have so far been restricted to laboratory mating experiments in Drosophila (references in Coyne and Orr 1989). In some cases, laboratory experiments may not directly reflect the extent of natural hybridization between sympatric species, particularly for those that are externally fertilizing (see discussion and references in McClary 1992). In such situations, reproductive isolation between sympatric species may be more appropriately assessed by natural hybridization frequency. However, analysis of natural hybrids is obviously limited to sympatric species, so some important evolutionary issues relating to reproductive isolation and hybridization (such as the existence of reinforcement) cannot be addressed. Recent genetic analyses of a sea star cryptic species complex (Leptasterias spp.) offer the possibility of testing the correlation between estimated degree of reproductive isolation in nature and divergence time for various pairs of sympatric mtDNA PCR-RFLP haplotypes. Occasional hybridization among members of this species complex had long been suspected (Fisher 1930; Kwast et al. 1990), but detailed analysis had to await the use of molecular polymorphisms as phylogenetic characters. Recent studies of this complex have uncovered at least 11 mtDNA PCR-RFLP haplotypes (denoted A-K in order of discovery), with many pairs that are broadly sympatric in Alaska, British Columbia, and the northwestern Pacific coast of the continental United States. Joint analysis of mtDNA and allozyme data suggests that the haplotypes constitute a species complex, similar to the sibling species complexes previously described in many other marine invertebrate taxa (Knowlton 1993). Genetic cohesiveness of each of the five most common and broadly distributed haplotypes (A, B, C, F, and G) was demonstrated in three ways: (1) principal components analysis (PCA) of multilocus allozyme genotypes within each haplotype revealed few outliers and no evidence for multimodal distributions of PCA scores (Foltz et al. 1996a); (2) Nei's (1978) unbiased genetic distances based on 14 allozyme loci were consistently smaller among population samples of haplotypes (range 0.001-0.045) than between haplotypes (range 0.086-0.198), leading to discrete clusters (Foltz et al. 1996a); and (3) direct sequencing of PCR products revealed negligible amounts of intrahaplotypic sequence polymorphism, compared to the level of interhaplotypic divergence (Hrincevich and Foltz 1996). Morphological differences between many pairs of mtDNA haplotypes were limited (especially for sympatric pairs), with statistically significant differences in canonical discriminant scores based on 55 characters but few diagnostic morphological characters (Foltz et al. 1996b), a pattern similar to those described for other marine invertebrate sibling species complexes (Knowlton 1993). The existence of numerous sympatric and closely related haplotypes within the Leptasterias cryptic species complex offered the possibility of studying the occurrence of hybridization between haplotypes in more detail. To test whether the extent of reproductive isolation increases with increased divergence time in this species complex, the frequency of putative F1 hybrids (as judged from the relative frequency of two-locus heterozygotes for informative allozyme alleles distinguishing the haplotypes) for sympatric pairs of haplotypes was compared to the amount of divergence estimated from mtDNA sequence data.