Cytogenetics of the Hawaiian Telmatogeton (Diptera)

Abstract

A cytogenetic analysis of the marine and freshwater species of the Hawaiian Telmatogeton was conducted to determine their evolutionary relationships. Telmatogeton torrenticola, which occupies the islands of Molokai, Maui, and Hawaii, is divided into separate species based on differences in chromosome number, karyotype, and fixed inversions. T. torrenticola-Molokai (n = 4) has three pairs of metacentric chromosomes and a pair of dot chromosomes. T. torrenticola-Hawaii (n = 3/4) has a differentiated sex chromosome system. Males have two pairs of metacentric autosomes, two acrocentric X-chromosomes, and a metacentric Y-chromosome. Females have two pairs of metacentric autosomes and two pairs of acrocentric X-chromosomes. T. torrenticola from East Maui and from West Maui have the same karyotype of six pairs of acrocentric and a pair of dot chromosomes, but they differ from each other by at least six fixed inversions. In a model of the evolution of the Hawaiian Telmatogeton it is proposed that a marine form was ancestral to the freshwater species. The model also proposes that one of these species may have returned to the sea. Chromosome evolution has involved reduction in chromosome number by centric fusion and the fixation of inversions. T. fluviatilis (n = 7) on Oahu is proposed to be the ancestral freshwater species. It gave rise to T. abnormis (n = 4) and T. hirtus (n = 3/4) on Kauai. T. fluviatilis also gave rise to T. torrenticola-Maui, the ancestor of T. torrenticola-Molokai (n = 4), T. torrenticola-E. Maui (n = 7), and T. torrenticola-Hawaii (n = 3/4). It is suggested that the marine species T. pacificus (n = 4) may have returned to the sea with T. torrenticola-Molokai as its ancestor since the two have identical karyotypes.