High levels of genetic variation in natural populations of marine lower invertebrates

Abstract

The predictions of neutralist and selectionist hypotheses have been tested many times in the past, but mostly using data only from organisms such as vertebrates, with generally low to average heterozygosities. The more recent discovery of particularly high levels of genetic variation in marine sponges and coelenterates provides an opportunity to use data from such species to contribute further to the understanding of the determinants of heterozygosity in natural populations. Therefore, 23 species of sponges and coelenterates from temperate, tropical and boreal waters were analysed by gel electrophoresis for an average of 14.3 enzyme loci per species.Mean heterozygosity values for each species were unusually high, ranging between 0.106 and 0.401. The means and variances of the heterozygosity estimates showed reasonable correlation with neutralist predictions (with both the stepwise mutation and the infinite alleles models). Population sizes were generally difficult to estimate with any confidence, but, for one sponge species for which this was possible, levels of heterozygosity again were similar to neutralist predictions, although the same was not apparently true for three species of sea anemone. No differences were found between heterozygosity levels of tropical and temperate species of sponges and coelenterates, thus apparently contradicting the selectionist 'trophic resource stability' and 'temporal environmental variation' hypotheses. Conversely, however, the consistently high levels of genetic variation found in coelenterates and sponges may be argued to be related to common biological characteristics, such as sessile life, great evolutionary 'age', limited ability to disperse and probable low homoeostatic capability.Our results seem, overall, to agree well with neutralist expectations for species with large, stable population sizes. Also, the mean heterozygosities, their variances and the observed and expected proportions of polymorphic loci seem to fit well with predictions based on the neutralist hypothesis. However, the selectionist 'environmental grain' and the 'shifting balance' hypotheses fit the data equally well. As with much earlier work, the problems in distinguishing between the various predictions of selectionist or neutralist ideas make it both difficult and unwise to draw definite conclusions.

The Main Protein of the Aggregation Factor Responsible for Species-specific Cell Adhesion in the Marine Sponge Microciona prolifera Is Highly Polymorphic

“…Allozyme variation in the tropical sponge Niphates erecta showed 84% correlation with the observed graft response between different individuals (69), although the enzymes used are not suspected to have any direct relation with cell/cell interaction systems. Along the same lines, the results of Curtis et al (70), reporting dissimilar plasma membrane proteins in graft-accepting pairs of sponges, are in agreement with the remarkable genetic variability of invertebrates (71,72) without being in conflict with the presence of a highly polymorphic histocompatibility system in sponges. Nevertheless, not all sponge genes are polymorphic, and single-copy polyubiquitin and fibrillar collagen genes have already been identified in G. cydonium (73) and E. mü lleri (67), respectively.…”

Genetic variation and species identification among selected leeches (Hirudinea) revealed by RAPD markers

“…Indeed, it evidences as high variation as 44 highly variable marine invertebrates with the mean polymorphism, P = 54% and H = 0.206 (Sole-Cava & Thorpe 1991). Likely, high RAPD polymorphism in G. complanata mirrors morphological diversity emphaBrought to you by | MIT Libraries Authenticated Download Date | 5/9/18 2:55 PM sized by colour variation observed in the population from Lake Ukiel (photos -supplementary data).…”

“…In all species the percentage of polymorphic loci (24%-58%) and gene diversity (0.099-0.219) fall within ranges recorded in about a thousand of animal and plant populations by means of isoenzymes. The polymorphism is from 10% to 50% whilst average diversities vary from 0.020 to 0.150 (Sole-Cava & Thorpe 1991). Leech genetic variation is also the same magnitude as in invertebrates, in which the mean coefficient of gene differentiation (H T -H S /H T ) is 0.171 with a range of 0.060-0.263 (Avise 2004).…”

Mitochondrial DNA Sequence Variation in a Sea Star (Leptasteriasspp.) Species Complex

“…No intrahaplotypic differences over the sequenced region were detected for haplotypes A, C, F, and H. This limited polymorphism was unexpected, given that intraspecific control region sequence differences of 1-2% had been found consistently in diverse vertebrate species (Kocher and Wilson, 1991;Bernatchez and Danzmann, 1993;Brown et al, 1993;Norman et al, 1994;Wenink et al, 1994). Also, haplotypes A, B, C, F, and G were shown by Foltz et al (1996b) to have allozyme heterozygosities averaged over 14 loci (range, 0.07-0.16) that were typical of marine invertebrate species (e.g., Sole-Cava and Thorpe, 1991). Finally, where possible, we selected individuals for sequencing from the extreme ends of the range of each haplotype, to maximize the chance of detecting sequence variants.…”