American Society of Ichthyologists and Herpetologists, Tampa, FL, July 2005.
Extensive introgression in Australian rainbowfishes: identification, patterns, implications, and misleading phylogenetic influence.
Introgression is increasingly being recognized as having a greater influence than previously appreciated in the evolution of fishes. Early evidence for natural hybridization was mostly restricted to F1 hybrids, often with little or no evidence for introgression between the parental species. The introduction of molecular analyses has begun to change this perception, with several groups showing evidence of mtDNA introgression including various Catostomidae, Cyprinidae, Cyprinodontidae, and other families. This study focuses on rainbowfishes (Melanotaeniidae), one of several dominant groups in Australian and New Guinea freshwaters. Rainbowfish hybridization was considered to be rare in natural populations due to extensive sympatry and the examination of thousands of specimens. The first mtDNA analysis found only limited evidence for hybridization within two species. Expansion of this work, based on mitochondrial cytochrome b and introns of the nuclear S7 gene has revealed evidence for extensive mtDNA introgression involving at least 10 of the 12 Australian Melanotaenia species. Introgressed populations from northwestern Australia were identified by the presence of mtDNA haplotypes from a sympatric species, whereas S7 haplotypes from the same specimens always formed monophyletic species-specific lineages that were strongly supported by bootstrapping. Numerous introgressed rainbowfish populations were identified, with some showing evidence of multiple introgressed haplotypes, suggesting either multiple hybrid origins, or post-introgressive evolution. For the most part, these introgressions appear to be short-term genetic influences, as no species appears to have completely lost its original mtDNA genome. However, one species has an introgressed mtDNA genome that has become extremely widespread and has clearly persisted for a longer time frame than other introgressions. Northwestern Australia provides a fascinating history of the evolution of rainbowfishes, and offers currently unparalleled opportunities for studying various aspects of introgressive hybridization, fitness differences of mtDNA genomes, sexual selection, and influences and limits to species reproductive boundaries.