Abstract
Group-living behavior is taxonomically widespread but rare in spiders. The conventional
view is that the main pathways to group-living in spiders are either sub-social, where
extended maternal care leads to prolonged sibling association; or communal living, where
individuals aggregate to exploit a common resource. Female egg-sac guarding behavior
occurs throughout kleptoparasitic spiders in the subfamily Argyrodinae (Theridiidae), while
individuals in group-living species cohabit in the resource rich webs of their host spiders.
These attributes fit both sub-social and communal routes to group-living, which offers new
insights to study the early stages of social evolution. We investigated whether members of
kleptoparasitic groups in natural populations comprise related individuals by comparing the
population structure of two group-living species, Argyrodes miniaceus and A. cf. fissifrons,
and two solitary species, A. fasciatus and Neospintharus trigonum. We found that: (1)
genetic-spatial autocorrelation in group-living species was highest among spiders sharing
the same host web and declined steeply with increasing distance, but no significant autocorrelation
at any scale for solitary species; (2) there was high relatedness among group members
in two cases of group-living species, which indicated relatedness was not an adhesive
agent in most of the groups, but no high relatedness in solitary species; and (3) the host web
boundary was not the sole predictor of genetic structures in group-living species. These
results suggest that population genetic structure in the group-living species is caused by limited
dispersal of group members that is favored by ecological conditions, including the
nature and size of resources. In contrast, the absence of genetic structuring in populations
of solitary species indicates a high level of dispersal with individual interactions unlikely to
have fitness benefits.