No abstract

Social spiders are unusual among social organisms in being highly inbred-males and females mature within their natal nest and mate with each other to produce successive generations. Several lines of evidence suggest that in spiders inbred social species originated from outbred subsocial ancestors, a transition expected to have been hindered by inbreeding depression. As a window into this transition, we examined the fitness consequences of artificially imposed inbreeding in the naturally outbred subsocial spider Anelosimus cf. jucundus. Subsocial spiders alternate periods of solitary and social living and are thought to resemble the ancestral system from which the inbred social species originated. We found that inbreeding depression in this subsocial spider only becomes evident in spiders raised individually following the end of their social phase and that ecological and demographic factors such as eclosion date, number of siblings in the group and mother's persistence are more powerful determinants of fitness during the social phase. A potential explanation for this pattern is that maternal care and group living provide a buffer against inbreeding depression, a possibility that may help explain the repeated origin of inbred social systems in spiders and shed light on the origin of other systems involving regular inbreeding.

We studied the temporal patterns of maturation and sexual receptivity of a subsocial spider, Anelosimus cf. jucundus, in southern Arizona. In subsocial spiders, sibling males and females share a common nest for a large portion of their life cycle, often only dispersing short distances close to the mating season. We found that, on average, male A. cf. jucundus matured 9 days earlier than females and that females did not become sexually receptive until 10 days following their final molt to maturity. The periods of sexual receptivity of sibling males and females, therefore, would be separated in time by a significant fraction of an adult male's life cycle. We also found significant asynchrony in maturation dates across nests and nest clusters at the two collection localities. We suggest that these temporal patterns may limit the opportunities for sibling males and females to mate with each other, thus explaining the apparent absence of mechanisms to discriminate against kin as mates in this species.

It has been recognized for some time that the risk of producing maleless clutches should select for a lower than binomial variance in the sex ratio of organisms with female-biased sex ratios, small clutches and breeding groups containing the clutch of a single female. However, to date, precise sex ratios have only been reported for organisms with haplodiploid sex determination, a system which allows direct control of the sex of individual o¡spring. In contrast, under heterogametic sex determination chance is expected to play a crucial role in determining the sex composition of any one family, in particular when males are the heterogametic sex. Here, we present evidence of precise or underdispersed primary sex ratios in the Neotropical social spider Anelosimus domingo Levi. We show that this diplodiploid species with male heterogamety has not only beaten the odds of meiosis by producing mostly daughters, but has also attained relative precision in the proportion of sons and daughters produced in any one clutch. The latter ¢nding suggests the existence of mechanisms that allow sorting of the two types of sperm in this spider species.