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Karyotypic characterization and nucleolus organizer regions in cyprinodon dearborni (MEEK, 1909) from Venezuela

Abstract:

Conventional Giemsa and silver staining techniques were used to analyze the chromosome complement of the "pupfish" Cyprinodon dearborni, using specimens from Margarita Island, Venezuela. This species is of potential interest because of its unique geographic distribution: All other pupfish species are Nearctic or Antellian, and no other Nearctic killiflsh genus (e.g. Fundulus) is represented on the South American mainland. The karyotype of our specimens comprises 1 metacentric (M) pair, 5 submetacentric (SM) pairs and 18 acrocentric (A) pairs, with an arm number of NF=60. Nucleolus organizer regions (NORs) are located terminally on the short arms of a medium subtelocentric pair. Sequential staining allowed to identify a homologous pair, designated as chromosome pair 9, as NOR-bearing, There may be a few arm ratio/shape differences between the karyotype of C. dearborni and those of congeneric species reported in the literature. However, the differences appear minor and may stem from variation in chromosome contraction related to fixation rather than real cytogenetic divergence. The present data extend the observation of apparent karyological conservatism in Cyprinodon to an unusual geographic "outlier" species. The clarity with which silver staining is able to resolve the NOR-bearing chromosome pair suggests that this technique may prove informative when applied to the chromosomes of congeneric species for higher resolution chromosome comparisons within the genus.