Molecular cytogenetics in artificial hybrid and highly polyploid sturgeons: An evolutionary story narrated by repetitive sequences.

We applied comparative genomic hybridization (CGH) and genomic in situ hybridization (GISH) to examine genomes of artificially produced sturgeon hybrids between sterlet, Acipenser ruthenus female (∼120 chromosomes) or Russian sturgeon, A. gueldenstaedtii female (∼240 chromosomes) and a spontaneous triploid Siberian sturgeon A. baerii male (∼360 chromosomes), respectively. The ploidy levels of progenies were analyzed by karyotyping and flow cytometry. We found that the species-specific regions were surprisingly identifiable only on some micro- and small(er) macrochromosomes in hybrid metaphases. We hypothesize that these distinguishable regions are represented by species-specific repetitive sequences driven by more dynamic molecular evolutionary mechanisms. On larger chromosomes, GISH faintly visualized only blocks of pericentromeric and telomeric repetitive sequences, remaining regions were equally shared by both parental species. We concluded that the interspecies hybridization producing viable and even fertile progeny is enabled by the fact that genomes of the species involved are likely divergent at the level of the repetitive sequences only and probably highly conserved in the coding sequences. These small differences of coding sequences are in concordance with previous estimations of relatedness of examined species producing artificial as well as natural hybrids. CGH and GISH represent a challenge in sturgeon cytogenetics as a valuable though technically not simple tool to discriminate chromosomes of parental species in hybrids. The potentials and drawbacks of CGH and GISH application in sturgeons are discussed and further experimental possibilities are proposed.

Chromosome studies of European cyprinid fishes: cross-species painting reveals natural allotetraploid origin of a Carassius female with 206 chromosomes.

A single female with 206 chromosomes and another 26 females with 156 chromosomes identified as Prussian carp, Carassius gibelio, and 5 individuals with 100 chromosomes identified as crucian carp, C. carassius, were sampled during field survey in one locality in the upper Elbe River. To identify the origin of females with high chromosome numbers, comparative karyotype analysis, GISH, with whole C. carassius DNA as probe and phylogenetic positions of sampled individuals revealed by cytochrome b mitochondrial marker were performed. GISH showed consistently bright labeling of 50 chromosomal elements out of 206, corresponding to the haploid chromosome number of C. carassius. The position of these females with high chromosome numbers in a reconstructed phylogenetic tree was within the clade of C. gibelio, documenting its affiliation to C. gibelio mitochondrial, i.e. maternal lineage. Our findings indicated that the mother of the female with high chromosome numbers was a gynogenetically reproducing 156-chromosome C. gibelio female and the father a bisexually reproducing C. carassius male. We, therefore, hypothesized that the C. gibelio × C. carassius allopolyploid female with 206 chromosomes arose by a mechanism of sperm genome addition to an unreduced egg of the mother.

Sexual intercourse involving giant sperm in Cretaceous ostracode.

Reproduction with giant sperm occurs in distinct groups scattered over the animal kingdom. Although experiments in Drosophila assessed the influence of different selection pressures on this character, no information was available on its long-term stability. Sub-micrometer-resolution synchrotron quantitative phase tomography (holotomography) of exceptionally well-preserved three-dimensional Cretaceous ostracode fossils from the Brazilian Santana Formation indicates that ostracode reproduction with giant sperm persisted for at least over the past 100 million years. Remnants of the male sperm pumps as well as giant, inflated female sperm receptacles evidence that, despite high costs, reproduction with giant sperm can be an evolutionary successful strategy.