Chromosome number and chromosome variation in embryos ofEvechinus 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 difference 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.

Brooding brittle-star is a global hybrid polyploid swarm

“…We will not review all mechanisms here but note examples that share similarities with the patterns found in A. squamata. Polyploidy has been rarely reported from echinoderms, although this may be due to the difficulty of undertaking cytogenetic studies in this phylum [8]. Polyploid cells were reported from the echinoid Evechinus chloroticus but considered likely to have resulted from laboratory induced polyspermy [8].…”

“…But research is almost completely lacking in phyla such as echinoderms and cnidarians which are known to exhibit varied asexual reproductive strategies [3]. The study of polyploidy in echinoderms has been hampered by practical problems in observing cytogenesis, including small chromosomes, tight clustering, low mitotic index and difficulties in obtaining meiotic preparations [8]. However, new DNA and RNA sequencing methodologies open an alternative path for research of polyploidism in echinoderms [e.g.…”

Karyotype diversity of pseudoscorpions of the genus Chthonius (Pseudoscorpiones, Chthoniidae) in the Alps

“…Tsurusaki 1985, Řezáč et al 2007, Schneider et al 2009), and many other organisms (e.g. Duffy et al 2008, Severns and Liston 2008, Dincǎ et al 2011, Sadílek et al 2013, Sember et al 2015). …”

Current Status of Echinoderm Genome Analysis - What do we Know?

“…The chromosomes of echinoderms have not been studied from the genomic or genetic point of view, although chromosome numbers have been reported since the end of the 19th century (see references in [ 28 ]). Echinoderm chromosomes are cytogenetically not easy to work with.…”

“…Therefore, various studies had yielded different chromosome numbers for the same species. Recent studies from all five classes of echinoderms (for example [ 28 - 33 ]), report that the majority of echinoderms possess between 36 and 46 chromosomes (diploid). Heteromorphic chromosomes were observed in mitotic figures from about half of the specimens of the sea urchins Paracentrotus lividus [ 31 ], S. purpuratus and Strongylocentrotus droebachiensis [ 30 ], and starfish Asterina pectinifera [ 33 ].…”