Eusocial evolution without a nest: kin structure of social aphids forming open colonies on bamboo

Living in nests is an almost universal feature of eusocial animals. In
some aphids, however, sterile soldier castes have evolved in open colonies
without a nest. To clarify the factors promoting the evolution of
eusociality in these colonies, we used newly developed microsatellite
markers to compare the kin structure of the open colonies of two aphid
species on bamboo: the non-eusocial colonies of Astegopteryx
bambucifoliae and the eusocial colonies of Pseudoregma
alexanderion Dendrocalamus latiflorus. Our samples, from
over 1,000 hectares, contained 99 clones of A. bambucifoliae and
19 of P. alexanderi. Clonal mixing occurred in both species:
average pairwise relatedness within a colony was 0.54 in A.
bambucifoliae and 0.71 in P. alexanderi. Each clone
of A. bambucifoliae occurred in a unique location,
whereas those of P. alexanderi occurred in multiple locations and
more than 90% of individuals came from just four clones. There was
significant genetic variation among different colonies in the same clump
(stem-cluster) in A. bambucifoliae but not
in P. alexanderi, indicating that P.
alexanderi colonies in a single clump are
genetically homogenized, functioning as a large colony. In P.
alexanderi, the proportion of sterile soldiers to normal first-instar
nymphs was significantly different across the four clones. Our
results indicate that the lack of input of migrants from the primary host
and feeding on a large, stable host plant are important ecological factors
that might favour the evolution of eusociality, enabling the production of
genetically homogenised, large, and long-lived colonies. After eusociality
evolves on the secondary host, the optimal strategy of soldier production
might vary between different clones.