Hybridization, reproductive isolation, and speciation in three Hexagrammos species

Because of the opposing effects of gene flow and speciation, hybridization and introgression have been considered interesting phenomena. Development of molecular genetic techniques have been important in the analysis of hybrid zones to characterize frequencies of pure species and hybrids, and to document patterns of gene exchange and introgression.;In the first chapter of my dissertation, I constructed a phylogeny of the family Hexagrammidae using multiple molecular loci. In addition, I wanted to verify species status of hybridizing Hexagrammos species. My data supported the species status of Hot, Hoc, and Hag, and these species exhibited fixed differences at six molecular markers.;With respect to the family Hexagrammidae, my results supported retention of some taxa traditionally classified in this family, but not others. I looked for defining criteria for family concepts, and found none. Consequently, I thought this would be an ideal opportunity to recommend criteria for introducing a family concept, and I used these criteria to evaluate the family Hexagrammidae.;In the second chapter of my dissertation, I genetically characterized relative proportions of hybrids, and patterns in parentage for Hexagrammos species at two distinct sites in the range of species overlap. Two classes of hybrids were observed in nature-between Hoc females and Hot or Hag males. The pattern of hybridization was asymmetric. All F1 hybrids sampled in nature were female. Thirteen backcrossed individuals were observed as adults, indicating potential for introgression.;In the third chapter of my dissertation, I wanted to determine the role of fitness costs associated with hybridization in processes of specdiation. To measure fitness components at several life history stages, I performed In vitro fertilization for all combinations of reciprocal hybrid crosses and backcrosses. I reared larvae for 30 day after hatching to estimate relative survivorship. There was clear evidence for fitness costs associated with hybridization at several life stages, including differences in gonadosomal indices between pure and hybrid adult females, reduced fertilization success, abnormal larval development, and larval survivorship from pure crosses, hybrid crosses, and backcrosses. Furthermore, trends in genomic interactions between the three hybridizing Hexagrammos species were apparent, and indicated implications for mechanisms of speciation.