Chromosome number and chromosome variation in embryos of Evechinus chloroticus (Echinoidea: Echinometridae): Is there conservation of chromosome number in the Phylum Echinodermata? New findings and a brief review.

Abstract

The recent publication of the genome sequence of the sea urchin, Strongylocentrotus purpuratus, has highlighted the deficiency of information about the cytogenetics of echinoderms. As the issue of numerical variation and chromosome stability in echinoderms has long been a source of dispute, the hypothesis of chromosomal stability needs to be tested with a wider range of species from diverse regions of the world. Here we have compiled a table of published echinoderm chromosome numbers and provide new data from the southern hemisphere echinoid species, Evechinus chloroticus. Analysis of E. chloroticus embryos taken from six different male-female urchin pairs showed a modal chromosome count of 2n = 42, as predicted by the chromosome stability hypothesis. However, a high degree of chromosomal variation was observed between individuals, with chromosome numbers ranging from 17 to 80. While there was no significant difference in mean chromosome number between all six parental combinations, there was a significant defference in mean chromosome number between embryos taken from two different urchin pairs for which intraembryo analysis was possible. These results indicate that extensive chromosomal variation can arise within embryos, and may also differ between individual urchin pairs. However, the extent to which methodological issues may contribute to the observed variation is unclear, and further research will be required to determine whether such variability is present within natural populations.