A comparative chromosome study of the North American species of sticklebacks (Teleostei: Gasterosteidae)

Abstract

The five North American species of the stickleback family Gasterosteidae were studied karyologically. They constitute two diploid-count groups; one with 46 includes Λpeltes quadracus and Culaea inconstans, and the other with 42 consists of Gasterosteus aculeatus, G. wheatlandi and Pungitius pungitius. The common occurrence of a distinctly large submetacentric pair in complements of all five species suggests a monophyletic origin of this character. The consistent presence of three additional large chromosome pairs in all members of the 2n = 42 group suggests their close relationship. Culaea probably possesses the closest karyotype to that of the progenitor of the 2n = 42 group. Apeltes is perhaps an offshoot of, and distantly related to, their common ancestor. The phylogenetic scheme constructed from the karyological data agrees with that constructed from grosmorphological and behavioral characters. The possible routes of chromosome evolution among the species and genera are discussed. Although pericentric inversions or reciprocal translocation or both might have achieved specific and generic differences, centric fusions or tandem translocations or both are required to separate the 2n = 46 group from the 2n = 42 group. Both the female Apeltes quadracus and the male Gasterosteus wheatlandi had a heteromorphic pair that was not detectable in the karyotype of the opposite sex. This may indicate a probable occurrence of cytologically expressed heterogamety in these two species.