Evidence for widespread cryptic sexual generations in apparently purely asexual Andricus gallwasps

Abstract

Oak gallwasps (Hymenoptera, Cynipidae, Cynipini) are one of seven major animal taxa that commonly reproduce by cyclical parthenogenesis (CP). A major question in research on CP taxa is the frequency with which lineages lose their sexual generations, and diversify as purely asexual radiations. Most oak gallwasp species are only known from an asexual generation, and secondary loss of sex has been conclusively demonstrated in several species, particularly members of the holarctic genus Andricus. This raises the possibility of widespread secondary loss of sex in the Cynipini, and of diversification within purely parthenogenetic lineages. We use two approaches based on analyses of allele frequency data to test for cryptic sexual generations in eight apparently asexual European species distributed through a major western palaearctic lineage of the gallwasp genus Andricus. All species showing adequate levels of polymorphism (7/8) showed signatures of sex compatible with cyclical parthenogenesis. We also use DNA sequence data to test the hypothesis that ignorance of these sexual generations (despite extensive study on this group) results from failure to discriminate among known but morphologically indistinguishable sexual generations. This hypothesis is supported: 35 sequences attributed by leading cynipid taxonomists to a single sexual adult morphospecies, Andricus burgundus, were found to represent the sexual generations of at least six Andricus species. We confirm cryptic sexual generations in a total of 11 Andricus species, suggesting that secondary loss of sex is rare in Andricus.

Fossil oak galls preserve ancient multitrophic interactions

“…(b) Oak gallwasps as indicators of past floras Andricus hungaricus lies within a clade of Andricus species whose lifecycles require obligate alternation between two different taxonomic sections of the genus Quercus (Cook et al 2002;Stone et al 2008). The asexual generation always galls section Quercus sensu stricto oaks (such as Q. robur and Quercus petraea), while the sexual generation always galls section Cerris oaks (particularly Turkey oak, Quercus cerris).…”

New Dryocosmus Giraud species associated with Cyclobalanopsis and non-Quercus host plants from the Eastern Palaearctic (Hymenoptera, Cynipidae, Cynipini)

“…Likewise, a significant proportion of other Holarctic cynipine species are known from only one of the two generations (Melika 2006). Stone et al (2008) confirmed that many of these species actually do have a sexual generation (Stone et al 2008) and it is likely that alternate generation of many East Asian Dryocosmus species are yet to be described. The above mentioned uncertainties about the distribution, host association and biology of Eastern Palaearctic Dryocosmus species demonstrate that research on this taxon is still in its infancy.…”

Partitioning of herbivore hosts across time and food plants promotes diversification in the Megastigmus dorsalis oak gall parasitoid complex

“…The same host traits could thus potentially also structure parasitoid populations. Each of the approximately 150 oak gall wasp species in the Western Palaearctic has one spring generation and one autumn generation per year (Stone et al., 2008), providing the potential for parasitoids to evolve seasonal phenologies that allow attack of one or both generations. Gall wasp generations also show high host‐plant specificity, and in the Western Palaearctic, almost all oak cynipid galls develop only on oak species from one (but not both) of the native oak sections Quercus or Cerris (Stone et al., 2009).…”