The Karyotypes of Galaxioid Fishes.

Abstract

Methods of cell treatment for metaphase chromosome spread analysis in teleost fish were evaluated. Once it was established that balancing the pH would allow galaxiid species to be kept in captivity for extended periods, the most reliable method proved to be the repeated excision of regrown fin borders. Good-quality spreads could be prepared from colchicine treated tissue pieces fixed in 3:1 ethanol:acetic acid and dissociated in dilute acetic acid. Chromosome arms were scored as present when they were consistently greater than 0.27 μm, irrespective of the arm ratio classification. Scanning electron microscopy of chromosomes stained with TCH ligated osmium did not give improved accuracy. Analysis showed the following: Galaxias argenteus 2n = 32 NF = 52; Galaxias brevipinnis 2n = 42 NF = 68; Galaxias divergens 2n = 42 NF = 58; Galaxias fasciatus 2n = 28 NF = 48; Galaxias gracilis 2n = 22 NF = 44; Galaxias maculatus 2n = 22 NF = 44; Galaxias postvectis 2n = 36 NF = 54; Galaxias vulgaris 2n = 44 NF = 80; Galaxiella munda (Australia) 2n = 28 NF = 50; Neochanna apoda 2n = 26 NF = 51; Neochanna burrowsius 2n = 44 NF = 76; Neochannn diversus 2n = 44 NF = 72; Retropinna retropinna 2n = 50 NF = 60; Lepidogalaxias salamandroides (Australia) 2n = 40 NF = 44. All the New Zealand Galaxias species except G. fasciatus and G. divergens appear to have four small metacentric chromosomes in the complement. Sperm morphology and ultrastructure were examined and described, and is consistent for G. divergens, G. fasciatus, G. gracilis, G. maculatus, G. vulgaris, and N. burrowsius. The ultrastructure of the retropinnid spermatozoon is also described from two species (Retropinna retropinna and Stokellia anisodon) and is consistent within the family, while being clearly different from that of the galaxiids. The DNA content was estimated from Feulgen stained blood smears and standardized against Salmo gairdneri. The results indicate that galaxiids and retropinnids have a DNA content 20% that of mammals, very close to that of the Osmeridae. A model of derivation for New Zealand galaxiid species is proposed, based on karyotype analysis, where some species (for instance G. divergens, and possibly G. vulgaris) have always been in fresh water and owe their distribution to tectonic events. The five diadromous species in New Zealand (G. argenteus, G. brevipinnis. G. fasciatus, G. maculatus, and G. postvectis) show clear similarities to species in Australia and appear to be derived from Australian freshwater ancestors. The acquisition of diadromy as a secondarily derived feature is proposed for some of the diadromous species - G. maculatus and possibly G. brevipinnis. A new hypothesis for the formation of the three species of Neochanna within New Zealand is proposed, where N. diversus and N. burrowsius are more closely related to each other than either are to N. apoda. Lepidogalaxias salamandroides shows no similarities in karyotype to any of the galaxiid fishes. The chromosomes of Retropinna retropinna show some similarities with those of the Osmeridae.