EVOLUTIONARY INTERACTIONS BETWEEN SEXUAL AND ALL-FEMALE TAXA OF CYPRINOTUS (OSTRACODA: CYPRIDIDAE)

Freshwater ostracodes show both an exceptionally high incidence of transitions to unisexuality and, in some cases, an extraordinary level of clonal diversity. There is no understanding of the agents promoting these transitions to thelytoky, although it has been suggested that their frequency may set the stage for sexual taxa to infuse clonal diversity into unisexuals. This study examines the nature and origins of clonal diversity in the unisexual ostracode Cyprinotus incongruens. A combination of allozyme and cytogenetic studies revealed the presence of two diploid clones of this species at three temperate sites and ten clones at one arctic site including three diploids, five triploids, and two tetraploids. The low heterozygosity (0%–20%) of its diploid clones suggests that parthenogenesis has arisen spontaneously in C. incongruens rather than through hybridization, as in vertebrate asexuals. Polyploid clones appear to owe their origin to genome additions from sexual taxa, although subsequent mutational divergence has played a role in further enhancing diversity. Two triploid clones have apparently originated from the incorporation of a haploid genome from the sexually reproducing C. glaucus, as evidenced by their high heterozygosity and possession of alleles otherwise found only in that species. Other polyploid clones have likely arisen as a result of interbreeding between bisexual and unisexual C. incongruens. These results suggest that both the incidence of spontaneous transitions to clonality and the frequency of interbreeding with relatives may be the key processes that govern clonal diversity in unisexual ostracodes.     

Influence of microbe-associated parthenogenesis on the fecundity ofTrichogramma deion andT. pretiosum

Microbe-associated parthenogenesis (thelytoky) has been discovered in nineTrichogramma species, parasitoids of mainly lepidopteran eggs. Parthenogenetic and bisexual conspecifics co-occur in many field populations. As an initial step to understand the dynamics of these two reproductive strategies we studied the effect of microbe-associated parthenogenesis on fecundity. The fecundity of two parthenogenetic isofemale lines ofT. pretiosum and one ofT. deion was compared with bisexual lines derived from them by antibiotic treatment. In all three cases parthenogenetic females were less fecund over their lifetime than bisexual females. Also, parthenogenetic females produced fewer daughters in two cases and in one case a similar number of daughters as their respective bisexual counterparts. The lack of mating and insemination was excluded as an explanation for the reduced fecundity of parthenogenetic females, because mated and virgin parthenogenetic females produce the same number of offspring. Antibiotic treatment can also be excluded because females of field-collected bisexual line treated with antibiotics produced the same number of offspring as untreated females. The reduced fecundity of parthenogenetic females was caused by a lower number of eggs being laid rather than by a greater developmental mortality. Parthenogenetic females produced less daughters than bisexual females when host availability was not limiting, but when host availability was severely limited, parthenogenetic females produced more daughters than the bisexual females.     

Maintenance and loss of heterozygosity in a thelytokous lineage of honey bees (Apis mellifera capensis)

An asexual lineage that reproduces by automictic thelytokous parthenogenesis has a problem: rapid loss of heterozygosity resulting in effective inbreeding. Thus, the circumstances under which rare asexual lineages thrive provide insights into the trade-offs that shape the evolution of alternative reproductive strategies across taxa. A socially parasitic lineage of the Cape honey bee, Apis mellifera capensis, provides an example of a thelytokous lineage that has endured for over two decades. It has been proposed that cytological adaptations slow the loss of heterozygosity in this lineage. However, we show that heterozygosity at the complementary sex determining (csd) locus is maintained via selection against homozygous diploid males that arise from recombination. Further, because zygosity is correlated across the genome, it appears that selection against diploid males reduces loss of homozygosity at other loci. Selection against homozygotes at csd results in substantial genetic load, so that if a thelytokous lineage is to endure, unusual ecological circumstances must exist in which asexuality permits such a high degree of fecundity that the genetic load can be tolerated. Without these ecological circumstances, sex will triumph over asexuality. In A. m. capensis, these conditions are provided by the parasitic interaction with its conspecific host, Apis mellifera scutellata.     

Antibiotic curing of parthenogenesis in Eretmocerus mundus (Australian parthenogenic form)

The thelytokous parasitoid, Eretmocerus mundus (Australian parthenogenetic form) was infected with a species of Group B Wolbachia. Antibiotic treatment of the parasitoid cured parthenogenesis and lead to male production and reduced egg hatching by virgin females. For each generation infected females produced more progeny than cured females. The descendants of individuals treated with antibiotic were in most cases free of a detectable infection. Both treated females and descendant females mated and received sperm, but these were not used as mated and unmated cured females produced offspring with a similar sex ratio. Males produced as a result of the antibiotic treatment were at least partially functional as they produced sperm and copulated with and inseminated females. However, no sexual line could be established.     

Effects of endosymbiotic Wolbachia on the diapause in Trichogramma hosts and effects of the diapause on Wolbachia

Abstract Used as an agent in biological control, Trichogramma which has been infected by endosymbiotic bacteria belonging to the genus Wolbachia and inducing the thelytokous mode of reproduction, increases rearing productivity in commercial insectaries. These insectaries can now make use of Trichogramma diapause to spread out and optimise their production. We therefore studied the effect of Wolbachia on the diapause in Trichogramma evanescens. The symbionts show no beneficial or harmful effect on the proportion of emergence in diapausing individuals or on their fecundity. Therefore, infected strains can easily be used in crop protection. On the other hand, low temperatures exert a slightly negative effect on Wolbachia, as detected from the proportion of females in the offspring of symbiotic diapausing females.     

Parthenogenesis in Hexapoda: holometabolous insects

This study is a second part of a review on parthenogenesis in Hexapoda. In contrast to the first part describing parthenogenetic groups of Entognatha and non‐holometabolous insects (Journal of Zoological Systematics and Evolutionary Research, 54, 2016, 257), this study deals with parthenogenesis in Holometabola. For each order of this group, the main parthenogenetic taxa and modes of parthenogenesis are listed. Data on parthenogenesis across Holometabola as well as other insects are summarized and discussed.     

MOLECULAR MARKERS INDICATE RARE SEX IN A PREDOMINANTLY ASEXUAL PARASITOID WASP

The parasitoid wasp genus Lysiphlebus (Hymenoptera: Braconidae: Aphidiinae) contains a taxonomically poorly resolved group of both sexual (arrhenotokous) species and asexual (thelytokous) clones. Maximum-parsimony and maximum-likelihood analyses of mitochondrial DNA sequence data from specimens collected across Western Europe showed that asexuality, which does not appear to be caused by the bacterium Wolbachia, is concentrated in two geographically widespread lineages, the older of which diverged from the closest extant sexual taxa approximately 0.5 million years ago. However, the DNA sequences of a nuclear intron (elongation factor—1α) showed no congruence with this pattern, and a much higher frequency of heterozygotes with very high allelic diversity was observed among the asexual females compared to that among females from the sexual species. This pattern is consistent with maternally inherited asexuality coupled with a history of rare sex with members of several closely related sexual populations or species. Our observations reinforce recent arguments that rare sex may be more important for the persistence of otherwise asexual lineages than hitherto appreciated.     

The life cycle of Micromalthus debilisLeConte (1878) (Coleoptera: Archostemata: Micromalthidae): historical review and evolutionary perspective

Micromalthus debilis 33 ), has one of the most bizarre life cycles of any metazoan. Reproduction is typically by thelytokous, viviparous, larviform females, but there is also a rare arrhenotokous phase. The active first‐instar (triungulin) larva develops into a legless, feeding (cerambycoid) larva. This form either pupates, leading to a diploid adult female, or develops into any of three subsequent types of reproductive paedogenetic forms: (1) a thelytokous female that produces triungulins via viviparity; (2) an arrhenotokous female that produces a single egg that develops into the short‐legged (curculionoid) larva, eventually devouring its mother and becoming a haploid adult male; or (3) an amphitokous female that can follow either of the two above reproductive pathways. We speculate that Micromalthus is dependent on maternally transmitted bacteria for the ability to digest rotting wood, and that these bacteria are senescent in males, causing males to be obligately cannibalistic. Obligate male cannibalism, in turn, would have dramatically increased the cost of males, and have created a strong selective advantage for cyclic thelytoky and the other features of the Micromalthus life cycle that minimize the role of the male.