Evolution of Clonality and Polyploidy in a Weevil System

Abstract

The increased interest in asexual organisms calls for in-depth studies of asexual complexes that actively give rise to new clones. We present an extensive molecular study of the Otiorhynchus scaber (Coleoptera, Curculionidae) weevil system. Three forms have traditionally been recognized: diploid sexuals, triploid, and tetraploid parthenogens. All forms coexist in a small central area, but only the polyploid parthenogens have colonized marginal areas. Analyzing the phylogenetic relationship, based on three partial mitochondrial genes, of 95 individuals from 19 populations, we find that parthenogenesis and polyploidy have originated at least three times from different diploid lineages. We observe two major mitochondrial lineages, with over 2.5% sequence divergence between the most basal groups within them, and find that current distribution and phylogenetic relationships are weakly correlated. Quite unexpectedly, we also discover diploid clones that coexist with, and are morphologically indistinguishable from, the diploid sexual females. Our results support that these diploid clones are derived directly from the diploid sexuals. We also find that it is mainly an increase in ploidy level and not the benefits of asexual reproduction that confers to polyploid parthenogens the advantage over their diploid sexual relatives.

Can resource costs of polyploidy provide an advantage to sex?

“…This idea has found support from studies such as that of Stenberg et al (2003), who contended that broad distribution of triploid vs. diploid asexual weevils suggested an intrinsic advantage of polyploidy (also see Adolfsson et al, 2010). More broadly, Stenberg et al (2003) presented evidence for 'geographical polyploidy,' a pattern analogous to geographical parthenogenesis, in which polyploid forms, regardless of sexuality, also tend to be found in higher latitudes (also see Suomalainen et al, 1987).…”

“…More broadly, Stenberg et al (2003) presented evidence for 'geographical polyploidy,' a pattern analogous to geographical parthenogenesis, in which polyploid forms, regardless of sexuality, also tend to be found in higher latitudes (also see Suomalainen et al, 1987). They argued that this pattern suggests that ploidy plays a key role in determining the distribution of many sexual and asexual taxa.…”

Genetic variability and adaptive evolution in parthenogenetic root-knot nematodes

“…So far, no evidence has been provided for alternative hypotheses about the origin of parthenogenesis in RKN, such as spontaneous origin by mutation in genes involved in the production of sexual forms, or due to infection by microorganisms such as Wolbachia. However, we cannot definitely exclude the possibility that there have been several different routes to apomixis in RKN species, as examplified by the insects Otiorhynchus scaber (Stenberg et al, 2003) and Rhopalosiphum padi .…”

What does the geography of parthenogenesis teach us about sex?

“…buttercup Ranunculus auricomus [3], weevil Otiorhynchus scaber [31]). Larger ranges seem instead more consistent with the second feature of many areas occupied by parthenogens: they have been more recently opened to colonization.…”