View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by University of Groningen University of Groningen Molecular Evidence for Ancient Asexuality in Timema Stick Insects Schwander, Tanja; Henry, Lee; Crespi, Bernard J. Published in: Current Biology DOI: 10.1016/j.cub.2011.05.026 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2011 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Schwander, T., Henry, L., & Crespi, B. J. (2011). Molecular Evidence for Ancient Asexuality in Timema Stick Insects. 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Download date: 25-12-2020 Current Biology 21, 1129–1134, July 12, 2011 ª2011 Elsevier Ltd All rights reserved DOI 10.1016/j.cub.2011.05.026 Report Molecular Evidence for Ancient Asexuality in Timema Stick Insects Tanja Schwander,1,2,3,* Lee Henry,1,2,4 and Bernard J. Crespi1 in Timema, ranging from less than 500,000 years to almost 1Simon Fraser University, Biological Sciences, Burnaby, 2,000,000 years (Figure 1). BC V5A 1S6, Canada The COI data for the asexual T. douglasi and its sexual sister T. poppensis also revealed that the nominal species T. douglasi [13] consists of at least three independently Summary derived asexual lineages, with different geographic distribu- tions. The phylogenetic trees for which we either constrained Asexuality is rare in animals in spite of its apparent advan- all T. douglasi sequences to be monophyletic, or split them tage relative to sexual reproduction, indicating that it must into two independent origins, differed significantly from the be associated with profound costs [1–9]. One expectation best maximum-likelihood (ML) tree in which T. douglasi was is that reproductive advantages gained by new asexual line- polyphyletic (Templeton tests, all p < 0.0001; Shimodaira- ages will be quickly eroded over time [3, 5–7]. Ancient Hasegawa tests, all p < 0.0001). By contrast, a constrained asexual taxa that have evolved and adapted without sex tree in which we defined three independent monophyletic would be ‘‘scandalous’’ exceptions to this rule, but it is often T. douglasi lineages (with individuals from the southern, difficult to exclude the possibility that putative asexuals central, and northern part of the distribution range, respec- deploy some form of ‘‘cryptic’’ sex, or have abandoned sex tively) did not differ significantly from the best tree (Templeton more recently than estimated from divergence times to test, p = 0.69; Shimodaira-Hasegawa test, p = 0.06). Mono- sexual relatives [10]. Here we provide evidence, from high phyly of the central and northern T. douglasi was also sug- intraspecific divergence of mitochondrial sequence and gested by the best ML tree (Figure 1; Figure S1). Although nuclear allele divergence patterns, that several indepen- we did not test for morphological or ecological divergence dently derived Timema stick-insect lineages have persisted among independently derived T. douglasi lineages, the without recombination for more than a million generations. sampled locations are suggestive of ecological divergence; Nuclear alleles in the asexual lineages displayed signifi- all northern and central T. douglasi were collected on redwood cantly higher intraindividual divergences than in related (Sequoia sempervirens), whereas the southern T. douglasi sexual species. In addition, within two asexuals, nuclear were almost exclusively found on Douglas fir (Pseudotsuga allele phylogenies suggested the presence of two clades, menziesii). Independently of putative ecological divergences with sequences from the same individual appearing in both among T. douglasi lineages, we overall had three asexual line- clades. These data strongly support ancient asexuality in ages of recent origin (T. shepardi and ‘‘central’’ and ‘‘south’’ Timema and validate the genus as an exceptional opportu- T. douglasi), one lineage of intermediate age (T. douglasi nity to attack the question of how asexual reproduction ‘‘north’’), and three potentially ancient asexual lineages can be maintained over long periods of evolutionary time. (T. genevievae, T. tahoe, and T. monikensis)(Figure 1). Divergence of Nuclear Alleles under Asexuality Results and Discussion Sexual and asexual reproduction are expected to generate different patterns of nuclear allele divergences and tree topol- High Levels of Intralineage Mitochondrial Divergence ogies [14–16](Figure 2). In a sexually reproducing diploid in Multiple Transitions to Asexuality species, recombination and genetic exchange between indi- The Timema walking-stick genus comprises at least five inde- viduals should tend to homogenize alleles within a population pendently derived asexual lineages (T. shepardi, T. douglasi, [14, 17]. Under asexuality, alleles should accumulate muta- T. genevievae, T. tahoe, and T. monikensis), all of which repro- tions independently of each other, a pattern also known as duce by functionally mitotic parthenogenesis [11, 12]. Three of the ‘‘Meselson effect’’ [14–16](Figure 2). However, neutral these, T. genevievae, T. tahoe, and T. monikensis, have been allelic divergence cannot always be used as an indicator of hypothesized to be older than the time frame predicted for ancient asexuality. For example, empirical studies revealed their extinction under asexual reproduction [11]. To improve that alleles in asexual Daphnia converge due to mitotic recom- available estimates of asexual lineage ages, we first bination and that the rate of conversion may be faster than sequenced a 771 bp portion of the mitochondrial COI gene in divergence due to mutation accumulation [18]. Conversely, 16–48 individuals of each asexual lineage and its closest Meselson effect-like allele divergence patterns may not neces- sexual relative from geographically widespread locations sarily indicate ancient asexuality. Most prominently, recent (see Table S1 available online; GenBank accession numbers hybridization between sexual species will generate allele HQ184475–HQ184697). This analysis corroborated previous divergences that mimic the patterns expected under long- evidence [11] for large-scale variation in asexual lineage age term asexuality, and many asexual lineages are known to derive from interspecific hybrids [3, 19–21]. Similarly, poly- ploidy and gene duplications could also generate Meselson 2These authors contributed equally to this work effect-like patterns, if paralogous loci instead of alleles at 3 Present address: Center for Ecological and Evolutionary Studies, Univer- a single locus are analyzed. sity of Groningen, Nijenborgh 7, 9700 CC Groningen, The Netherlands 4Present address: Zoology Department, University of Oxford, The Tinbergen To develop insights into nuclear allele divergences under Building, South Parks Road, Oxford OX1 3PS, UK asexuality and to test for ‘‘ancient asexuality’’ in Timema,we *Correspondence: [email protected] analyzed allelic variation at Actin and Hsp70 in each of the Current Biology Vol 21 No 13 1130 Figure 1. High Levels of Intralineage Mitochondrial Sequence Divergence in Asexual Timema (A) Asexual lineage age estimates; sequence divergences were converted to ages with the approximation of 2% divergence per million years in insect mtDNA [39, 40]. The upper limits (horizontal bars) are based on the divergences (simple p-distances) from their sexual sister species, lower limits (triangles) on intraspecific divergence. The interspecific divergences are based, in each case, on the closest sexual haplotype(s), not the entire sexual sister species. Note that the similar levels of intra- and interspecific divergences in T. genevievae and T. douglasi (north), in combination with the monophyly of these groups in the best ML tree indicate that their ages are unlikely to be overestimated because of a missing sexual sister population. In contrast, intralineage diver- gence in T. douglasi (south) may be overestimated if this group comprises multiple, independently derived asexual lineages. (B) Simplified best ML phylogeny; for the complete phylogeny with branch support values, see Figure S1. The sizes of the triangles are proportional to the sequence diversity in a given clade. seven asexual lineages (for a total of 25 individuals for Actin In contrast, and in spite of the low sequence variability, we and 20 for Hsp70). For comparison, we also analyzed allele found evidence for recombination in three sexual species: divergence patterns in the sexual sister species of the asexual T. cristinae (at least one recombination event for both Actin lineages (T. poppensis, T. californicum, T. cristinae, T. podura, and Hsp70), T. californicum (Actin,