Origins of asexuality in Bryobia mites (Acari: Tetranychidae)

Vera ID Ros 0 Johannes AJ Breeuwer 0 Steph BJ Menken 0 0 Address: Evolutionary Biology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam , P.O. Box 94062, 1090 GB Amsterdam , The Netherlands Background: Obligate asexual reproduction is rare in the animal kingdom. Generally, asexuals are considered evolutionary dead ends that are unable to radiate. The phytophagous mite genus Bryobia contains a large number of asexual species. In this study, we investigate the origin and evolution of asexuality using samples from 111 populations in Europe, South Africa and the United States, belonging to eleven Bryobia species. We also examine intraspecific clonal diversity for one species, B. kissophila, by genotyping individuals from 61 different populations. Knowledge on the origin of asexuality and on clonal diversity can contribute to our understanding of the paradox of sex. Results: The majority (94%) of 111 sampled populations reproduces asexually. Analysis of part of nuclear 28S rDNA shows that these asexuals do not form a monophyletic clade. Analysis of the mitochondrial COI region shows that intraspecific variation is extensive (up to 8.8%). Within B. kissophila, distinct clades are found, which are absent at the nuclear 28S rDNA level. Moreover, paraphyletic patterns are found at the mitochondrial DNA. Conclusion: Asexuality is widespread in the genus Bryobia, signifying that some animal taxa do contain a high number of asexuals. We argue that asexuality originated multiple times within Bryobia. Wolbachia bacteria cause asexuality in at least two Bryobia species and may have infected different species independently. The high intraspecific clonal diversity and the patterns of paraphyly at the mitochondrial DNA in B. kissophila might be explained by a high mutation fixation rate and past hybridization events. Reproductive parasites like Wolbachia and Cardinium might influence these processes. We discuss the role these bacteria could play in the evolutionary success of asexual species. - Background Asexual taxa are found across the eukaryotic tree of life: many plant, fungal, and animal taxa contain asexual lineages. In most cases however, this asexuality is facultative: apparent asexual species do have sex now and then. Obligate less than 1% of all species reproduce strictly asexually [1]. The distribution of these asexuals is 'tippy': most asexuals are found as single branches on the tips of the tree, branching off from closely related sexual species [1,2]. Fully asexual taxa contain few species. Apparently, asexual species can survive in the short-term, but are doomed to extinction in the long-term. Asexual reproduction has short-term advantages compared to sexual reproduction. In a sexual population females will produce both sons and daughters, but in an asexual population females will produce only daughters. An asexual population has therefore twice the growth rate of a sexual population (assuming a sexually reproducing population with an equal sex ratio and with males contributing nothing but gametes to the offspring). In other words, there is a two-fold cost of sexual reproduction [3,4]. Besides this two-fold cost, there are other costs related to sexual reproduction: costs of finding a mate, of sexually transmitted diseases or selfish genetic elements, or of the act of sex itself [5]. Despite these costs, sex is widespread (the 'paradox of sex'). General explanations for this paradox refer to the long-term disadvantages of asexual reproduction: asexuals are less able to adapt to novel environments and are exposed to accumulation of deleterious mutations (reviewed in [1,6-10]). This is why asexuals are considered short-lived evolutionary 'dead ends' with limited adaptive potential [1,11,12]. It also explains the sporadic and low-level phylogenetic distribution of obligate asexual lineages. An exception to this pattern are a few groups that have been reproducing exclusively asexually for a long evolutionary time, like the bdelloid rotifers [13], darwinulid ostracods ([14]; but see [15]), and oribatid mites [16,17]. The phytophagous spider mite genus Bryobia (Acari: Tetranychidae) contains both asexually (thelytokous) and sexually (arrhenotokous) reproducing species. The genus is poorly studied and phylogenetic relationships are unknown. Species are described on the basis of morphology and host plant associations [18,19]. However, suitable morphological characters are rare in these tiny mites and this severely limits identification. In addition, host plant associations are generally considered unsuitable as primary input for species identification [20]. Bolland et al. [21] list over 130 species names, but these are likely to include synonyms and overlapping species descriptions, as such descriptions are often based on morphological descriptions of quantitative characters (e.g., body size, number and length of setae) in single, locally occurring mites. Nonetheless, for the majority of described species, no males have been reported and females reproduce asexually through thelytokous parthenogenesis [22], indicating that asexuality is widespread in this genus. Parthenogenesis in at least two asexual Bryobia species is caused by the bacterial endosymbiont Wolbachia [23]. Wolbachia are reproductive parasites that enhance their own transmission by manipulating the reproduction of their host, resulting in an increased number of infected females (see [24] for a review). Wolbachia were detected in four additional asexual Bryobia species, but the causal effect was not established [23]. In addition, Weeks and Breeuwer [23] showed that the parthenogenesis is functionally apomictic, as heterozygosity is maintained. The occurrence of many asexuals in one genus is rare and raises questions about the origin and evolution of the asexual lineages. One way to address such questions is using a phylogenetic approach [25]. Suppose a phylogeny shows that asexuals occur as single lineages among sexual sister groups, indicating that they are 'evolutionary dead ends' that are unable to radiate. Then, the most likely explanation is multiple (and recent) origins of asexuality. On the contrary, if all asexuals form a monophyletic group, the most likely explanation would be a single and older origin with subsequent radiation of asexuals, a phenomenon that has rarely been found for asexuals [2]. In this study, we investigate the phylogenetic history of asexual reproduction in the genus Bryobia. Also, we examine intraspecific clonal variation by analyzing samples collected on a large geographic scale. Generally, clonal species are thought to harbor little genetic diversity. This approach provides the framework for investigating the evolution of asexuality and host plant specificity across the genus. We use a combination of mitochondrial (the cytochrome oxidase c subunit I gene, COI) and nuclear (the 28S rDNA gene) sequence data for inferring species relationships (...truncated)