Genomic divergence of allopatric sibling species studied by molecular cytogenetics of their F1 hybrids

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Abstract
Despite their similar karyotype morphology and close taxonomic affinity, the genomes of allopatric sibling species, Gibasis karwinskyana and Gibasis consobrina, are clearly distinguished in metaphases of their F1 hybrids by genomic in‐situ hybridization (GISH). The reduced ability of chromosomes from one species to bind labelled total DNA from the other involves almost the whole chromosome complement, and is equally pronounced in euchromatin and heterochromatin. The only region strongly conserved in the two species is an AT‐rich band proximal to each nucleolus organizer. Molecular differentiation is accompanied by chromosome pairing failure in the F1 interspecific hybrids, although the reason remains open to question. The two species also differ in their numbers of detectable sites for rRNA genes. The greater number of such sites in G. consobrina may be linked with a propensity for interchange heterozygosity. The ability to discriminate rapidly and reliably between the chromosomes of close relatives with almost identical karyotypes makes GISH invaluable in preliminary studies of phytogeny. Detection of even small conserved chromosome bands using GISH confirms the sensitivity of the technique and demonstrates its potential use in evolutionary cytogenetics. This will allow rapid re‐evaluation of many important genetic systems exposed by classical cytogenetics in previous decades.