Aneuploidy Occurrence in Oligochaeta

Abstract

Appearance of aneuploidy in the germ and somatic lines is usually associated with chromosome and genome rearrangements leading to polysomies and cancer. However, aneuploidy plays an important role in chromosome evolution and in the regulation of the ontogenetic development and phenotypic expression. The latter is known as chromosome diminutions. In Oligochaeta (mainly family Naididae but also Lumbricidae, Erpobdellidae and Branchiobdellidae), we have equated the variability of the chromosome count numbers with aneuploidy based on the results of our analyses and identified chromosome-like nondisjunctions as a major mechanism responsible for it. Another author detected Robertsonian-like translocations producing aneuploidy in Eisenia fetida (Lumbricidae, Oligochaeta). Our observations, nevertheless, show that, among karyotyped haploid/diploid cells, the most frequent were haploid (1n) or diploid (2n) chromosome counts connected by multiples. The number of aneuploidy counts was decreasing with the increase of x in expressions 1n + x/1n – x or 2n + x/2n – x. Noteworthy is that not all frequencies of chromosomes in a pair have the same probability. For example, odd aneuploidy numbers of chromosomes are significantly less frequent than the even ones. The wide spread of aneuploidy among Oligochaeta supports the punctuated equilibria model of evolution.