Cytogenetic analysis of a self-fertilizing fish, Rivulus marmoratus: remarkable chromosomal constancy over a vast geographic range

Abstract

The aplocheiloid killifish Rivulus marmoratus is the only known self-fertilizing hermaphroditic vertebrate. Most natural populations consist almost entirely of hermaphrodites and comprise arrays of homozygous clones. However, in almost all populations thus far studied, clonal variation, as detected with molecular techniques, is very high. A karyological survey was carried out on specimens from Brazil, the Bahamas, Belize, and Florida (4 locales) by C-banding, silver staining, and fluorescent staining. The chromosome complement of R. marmoratus is surprisingly uniform over its vast geographic range, in terms of both chromosome number and morphology, heterochromatin distribution and composition, and nucleolar organizer region (NOR) distribution. The short arms of chromosome pair 15, where NORs are located, showed the only variation detected in this study: those of pattern A were consistently shorter than those of pattern B; moreover, the latter show positive heteropycnosis with Giemsa staining. The present data demonstrate that chromosomal variation is not a significant part of the clonal divergence in this species, even though its breeding system, by forming homozygotes for new rearrangements almost immediately, should make that variation easier to detect. The high chromosomal homogeneity is discussed in the light of the peculiar natural history of the species.

Differential Expression of Conserved Germ Line Markers and Delayed Segregation of Male and Female Primordial Germ Cells in a Hermaphrodite, the Leech Helobdella

“…Microsatellite analysis of four Kryptolebias species indicates that the capacity for self-fertilization in K. marmoratus and K. ocellatus has been maintained for several hundred thousand years or more (Tatarenkov et al 2009); analysis of locally most common multilocus genotypes suggests that fertilization assurance rather than mulitlocus coadaptation is the driving force favoring self-fertility in this species (Avise and Tatarenkov 2012). Kryptolebias marmoratus shows chromosomal constancy over a large geographic range, but whether this represents recent, rapid spread or selection against the expected rearrangements cannot be determined (Sola et al 1997). …”

Meristic and Morphometric Differences in Populations of Rivulus marmoratus

“…Several lines of evidence point to R. •marmm~ atus as having recently diverged from an ancestral form. Sola et al (1997) deduced from the nearly uniform karyotypes (both number and morphology) of R. marnwratus from four disparate locales [Brazil, Turks and Caicos (Bahamas), Belize, and Florida (four sites)] that the species is "young" and that chromosomal variation has not yet occurred. Murphy (1997) implies that R. mannomtus descended directly from R. caudomarginatus and that R. nwrmoratus has since spread rapidly north from southeast Brazil.…”

Vasa Identifies Germ Cells and Critical Stages of Oogenesis in the Asian Seabass

“…In many invertebrates and lower vertebrates such as fish, hermaphroditism is commonplace, with one organism developing into both sexes. Simultaneous hermaphrodites have both the ovary and testis and thus produce eggs and sperm at the same time, such as Rivulus marmoratus 11. Sequential hermaphrodites produce sperm and eggs or vice versa in different times.…”