CYTOGENETIC ANALYSIS OF A SELF-FERTILIZING FISH, RIVULUS-MARMORATUS -REMARKABLE CHROMOSOMAL CONSTANCY OVER A VAST GEOGRAPHIC RANGE

The aplocheiloid killifish Rivulus marmoratus is the only known self-f ertilizing hermaphroditic vertebrate. Most natural populations consist almost entirely of hermaphrodites and comprise arrays of homozygous c lones. However, in almost all populations thus far studied, clonal var iation, as detected with molecular techniques, is very high. A karyolo gical survey was carried out on specimens from Brazil, the Bahamas, Be lize, and Florida (4 locales) by C-banding, silver staining, and fluor escent staining. The chromosome complement of R. marmoratus is surpris ingly uniform over its vast geographic range, in terms of both chromos ome number and morphology, heterochromatin distribution and compositio n, and nucleolar organizer region (NOR) distribution. The short arms o f chromosome pair 15, where NORs are located, showed the only variatio n detected in this study: those of pattern A were consistently shorter than those of pattern B; moreover, the latter show positive heteropyc nosis with Giemsa staining. The present data demonstrate that chromoso mal variation is not a significant part of the clonal divergence in th is species, even though its breeding system, by forming homozygotes fo r new rearrangements almost immediately, should make that variation ea sier to detect. The high chromosomal homogeneity is discussed in the l ight of the peculiar natural history of the species.