Current reproductive isolation between ancestors of natural hybrids in Bacillus stick insects (Insecta: Phasmatodea)

Laboratory generation of new parthenogenetic lineages supports contagious parthenogenesis in Artemia

Contagious parthenogenesis—a process involving rare functional males produced by a parthenogenetic lineage which mate with coexisting sexual females resulting in fertile parthenogenetic offspring—is one of the most striking mechanisms responsible for the generation of new parthenogenetic lineages. Populations of the parthenogenetic diploid brine shrimp Artemia produce fully functional males in low proportions. The evolutionary role of these so-called Artemia rare males is, however, unknown. Here we investigate whether new parthenogenetic clones could be obtained in the laboratory by mating these rare males with sexual females. We assessed the survival and sex ratio of the hybrid ovoviviparous offspring from previous crosses between rare males and females from all Asiatic sexual species, carried out cross-mating experiments between F1 hybrid individuals to assess their fertility, and estimated the viability and the reproductive mode of the resulting F2 offspring. Molecular analysis confirmed the parentage of hybrid parthenogenetic F2. Our study documents the first laboratory synthesis of new parthenogenetic lineages in Artemia and supports a model for the contagious spread of parthenogenesis. Our results suggest recessive inheritance but further experiments are required to confirm the likelihood of the contagious parthenogenesis model.

Facultative sex and reproductive strategies in response to male availability in the spiny stick insect, Extatosoma tiaratum

Facultative thelytoky, in which females can reproduce both sexually and asexually, offers a promising model system to understand the evolutionary significance of sex, by providing insights into whether the different reproductive modes reflect an adaptive life-history response to varying environmental conditions. Females of the spiny stick insect, Extatosoma tiaratum, can reproduce both sexually or asexually. We show that virgin females signal their reproductive state: males respond to signals produced by virgin females that have not commenced ovipositing, but fail to respond to ovipositing virgin females. Virgin females reared under different social environments varied their reproductive output: virgin females reared in the absence of males laid more eggs over a seven-day period than virgin females reared in the presence of males. The reproductive output of mated females over a seven-day period was higher than that of virgin females. These data suggest that female E. tiaratum adjust several life-history strategies in conjunction with facultative thelytoky.

Parasites and sexual reproduction in psychid moths

Persistence of sexual reproduction among coexisting asexual competitors has been a major paradox in evolutionary biology. The number of empirical studies is still very limited, as few systems with coexisting sexual and strictly asexual lineages have been found. We studied the ecological mechanisms behind the simultaneous coexistence of a sexually and an asexually reproducing closely related species of psychid moth in Central Finland between 1999 and 2001. The two species compete for the same resources and are often infected by the same hymenopteran parasitoids. They are extremely morphologically and behaviorally similar and can be separated only by their reproductive strategy (sexual vs. asexual) or by genetic markers. We compared the life-history traits of these species in two locations where they coexist to test predictions of the cost-of-sex hypothesis. We did not find any difference in female size, number of larvae, or offspring survival between the sexuals and asexuals, indicating that sexuals are subject to cost of sex. We also used genetic markers to check and exclude the possibility of Wolbachia bacteria infection inducing parthenogenesis. None of the samples was infected by Wolbachia and, thus, it is unlikely that these bacteria could affect our results. We sampled 38 locations to study the prevalence of parasitoids and the moths' reproductive strategy. We found a strong positive correlation between prevalence of sexual reproduction and prevalence of parasitoids. In locations where parasitoids are rare asexuals exist in high densities, whereas in locations with a high parasitoid load the sexual species was dominant. Spatial distribution alone does not explain the results. We suggest that the parasite hypothesis for sex may offer an explanation for the persistence of sexual moths in this system.

Chromosomal and NOR patterns in the polyclonal stick insect Bacillus atticus atticus (Insecta; Phasmatodea)

Bacillus atticus atticus is a complex of thelytokous parthenogens, related to the bisexual Bacillus grandii, that ranges from Sardinia to Near Eastern countries. Karyotypic and cytogenetic differentiation of the B. atticus atticus diploid unisexual "isolates" is really higher than expected. Its standard karyotype has 2n = 34 chromosomes, but several instances of repatterned or even aneuploid complements have been found. The number and location of silver-stained NORs are particularly intriguing, since in addition to homozygous NOR patterns, simple or double hemizygous strains are found spread over specific and wide regions. The odd patterns are not due to Ag-NOR staining technique artifacts, since the FISH method, using rDNA probes, apparently labels the same ribosomal clusters. Transpositions and translocations have been suggested to account for some NOR patterns, but hybridizations between different NOR-bearing races are also a possible cause. This chromosomal survey clearly contributes to a better understanding of B. atticus phylogeny.Key words: aneuploidy, interracial hybrids, karyotype repatterning, NOR techniques, unisexuals.