Deviation from apomictic reproduction in Dendrobaena octaedra?

Abstract

The earthworm Dendrobaena octaedra has an original distribution covering the central and northern Palearctic but has spread over large areas of North America and has caused considerable concern there as the introduction may lead to collapse of local species (CALLAHAM et al. 2006; TIUNOV et al. 2006). It is reported to reproduce through apomictic parthenogenesis (OMODEO 1955). This is in contrast to all other parthenogenetic earthworms that have automictic (meiotic) parthenogenesis (SUOMALAINEN et al. 1987). The number of chromosomes and the morphology of the internal and external male organs may vary (OMODEO 1955; CASELLATO and RODIGHIERO 1972; VEDOVINI 1973) but D. octaedra in northern Europe are hexaploids with 6n 108. This number was confirmed by a subsequent study (HONGELL and TERHIVUO 1989). When doing isozyme analysis (TERHIVUO and SAURA 1990) it was observed that the clonal diversity was high, among 428 individuals 147 clones based on six enzymes were observed and of these 80 clones were unique, i.e. only found in a single individual. This high level of clonal diversity was also found in a study done in the northern hemisphere (HANSEN et al. 2006) where 176 clones based on seven enzymes, at least representing nine loci revealed 125 unique clones among 345 worms analysed. Compared to other parthenogenetic earthworm species in northern Europe, D. octaedra always has a very high level of clonal diversity (TERHIVUO and SAURA 2006). The high diversity may be due to efficient dispersal of clones, effects of polyploidy, modification of gene products (proteins), meiotic thelytoky accompanied with segregation, self fertilisation or occasional sexual reproduction (cf. BENGTSSON 2003). However, self fertilisation or sexual reproduction can most likely be excluded since only incomplete spermatogenesis is found (OMODEO 1955, 1957). Furthermore, absence of male pores in many specimens does not support biparental reproduction or self fertilisation (TERHIVUO 1988; TERHIVUO and SAURA 2006). In an ovum from the ovisac 108 chromosomes were found to be univalents (HONGELL and TERHIVUO 1989) which indicated that oogenesis was asynaptic, i.e. the eggs were developing through mitosis instead of meiosis, which is an argument against meiotic thelytoky. This observation fits to earlier findings (OMODEO 1955). Only high rates of dispersal, effects of polyploidy in combination with null alleles and/or segregation distortion or modifications of gene products may explain the high level of diversity. In the present study a comparison of phenotypes from reproducing individuals and their offspring by means of allozymes was carried out to see if the apomictic parthenogenetic reproduction mode could be confirmed.