QUESTIONS AND ANSWERS

The evolution of sexual isolation in sticklebacks.
Jenny Boughman, Univ. of Wisconsin

Q:  What evidence supports the idea of the double introduction as opposed to sympatric speciation? Fossil evidence, molecular clock estimates? Homology of genes to marine sticklebacks? Are your "ecotypes" all different species?

A: The species are too recent for fossil evidence. Instead the evidence is from a combination of geological history of the area and neutral markers. The geological history indicates there were two marine incursions which would have allowed two invasions of the marine stickleback into the lakes. All the lakes that have pairs are low elevation and in the region where this occurred. Two separate lines of genetic evidence indicate that the species pairs from these three lakes are the result of separate colonization events of the marine stickleback into freshwater (Rundle & Schluter 2004), as originally proposed by (McPhail 1993). The first comes from mtDNA (Taylor & McPhail 1999). The limnetic-benthic pairs within each lake are characterized by unique assemblages of mtDNA haplotypes. The majority of these differ from common marine haplotypes by a single restriction site. In contrast, mtDNA haplotypes from different lakes always differ by more than one site. These patterns suggest that the separate assemblages in each lake trace their origin independently to the marine environment and not to each other. The second line of evidence comes from an analysis of allelic variation at six microsatellite loci (Taylor & McPhail 2000). If limnetics and benthics each arose only once and then colonized these lakes, genetic variation should be structured into 'limnetic' and 'benthic' classes. However, almost none of the variation (2.2-4.4%, which is not significantly different from zero) can be partitioned between these classes. In addition, although poorly resolved, the maximum-likelihood phylogeny suggests independent origins for limnetics and benthics from these lakes and is a significantly better fit to the data than one enforcing monophyly of either.

Q: Has any post-zygotic reproductive isolation evolved for these populations (within lake) or are hybrids formed? If hybrids occur, are they capable of reproducing?

A: There is no evidence of intrinsic postzygotic isolation--artificial crosses result in viable, fertile offspring. However, there is substantial evidence for extrinsic postzygotic isolation. Basically hybrids are intermediate and fall between niches, so they are inferior to both parentals and have reduced fitness. There is also some evidence for sexual selection against hybrids.

Q: Are there predators in these lakes? How does predation influence the evolution of these colors?

A: Yes, of course there are predators, but we don't yet know how predation influences color and body size, or the differences in these traits between species. We do know that each species is exposed to a different suite of predators, has different antipredator adaptations, and each survives best w/ its own predators.

Q: Because your lab studies include trials with 1 female and 1 male, is there any possibility that competition between males in their natural habitat could lead to different conclusions? What are the sensitivities of males and their ability to see each other? Does male perception of color matter?

A: There is a chance that male competition occurs and might affect which males have a chance to court females. We have experiments planned to look at this. We don't have data on male color perception yet, but these experiments are also planned. We do know from other sticklebacks (especially marine populations) that color influences male encounters.