Inferring dispersal of aquatic invertebrates from genetic variation: A comparative study of an amphipod, Talitridae Hyalella azteca, and mayfly, Baetidae Callibaetis americanus, in Great Basin springs

Authors: Heather L. Stutz, Dennis K. Shiozawa

Year: 2009 (xli)

Abstract

Dispersal studies often ignore the relevant impact of historical events. Whether dispersal is active or passive, dispersal accompanied by gene flow shapes the population genetics and evolutionary divergence of species. Indirect methods which use genetic markers have the ability to assess effective dispersal—that which resulted in gene flow. Two spring invertebrates with differing dispersal abilities were evaluated for genetic variation. The amphipod, Hyalella azteca, and mayfly, Callibaetis americanus, were collected from 4-5 springs in each of six basins in the Great Basin of western North America. No dispersal or genetic studies of C. americanus have been conducted to date. However, several studies focusing on mtDNA diversity of H. azteca have revealed a tremendous degree of cryptic diversity in the desert springs of the Great Basin. My objective was to test if aquatic insects that do not have the same restraints as obligate aquatic invertebrates show similar phylogeographic patterns and genetic uniqueness. I hypothesized the amphipods would be genetically differentiated populations amongst springs, and the mayflies would be more genetically homogeneous amongst springs. Nested clade phylogeographical analysis (NCPA), FST values, AMOVA, and Mantel tests were used to examine geographical associations. I also used traditional phylogenetic approaches including maximum parsimony (MP) and likelihood (ML) analyses using cytochrome c oxidase subunit I (COI), 28S, and 16S as genetic markers. The mitochondrial COI sequence divergences in C. americanus were higher than H. azteca COI divergences within springs but lower among springs. FST values were very high in H. azteca reaching near fixation for certain alleles. C. americanus FST values were lower suggesting greater gene flow and, consequently, greater dispersal rates. Even though Mantel tests did not detect significant isolation by distance when evaluating all haplotypes together, nested clade analysis was able to examine smaller networks of related haplotypes and detect significant isolation by distance. Whereas the genetic structure in C. americanus was dominated by restricted gene flow with isolation by distance, H. azteca was characterized more by gradual range expansion followed by fragmentation. Mayflies likely showed more gene flow than amphipods because of their flight capabilities, but movement was still restricted by long distances between isolated springs.