Group-housed females promote production of asexual ootheca in American cockroaches.
BACKGROUND
Facultative parthenogenesis, seen in many animal phyla, is a reproductive strategy in which females are able to generate offspring when mating partners are unavailable. In some subsocial and eusocial insects, parthenogenesis is often more prevalent than sexual reproduction. However, little is known about how social cooperation is linked to the promotion of parthenogenesis. The domiciliary cockroachPeriplaneta americanais well-suited to addressing this issue as this species belongs to the superfamily Blattoidea, which diverged into eusocial termites and shows facultative parthenogenesis.


RESULTS
We studied environmental factors that influence asexual production of ootheca using behavioral assays inP. americana. When more than three virgin females immediately after the imaginal molt were kept together in a small sealed container, they tended to produce egg cases (oothecae) via parthenogenesis earlier than did isolated females, resulting in apparent synchronization of ootheca production, even among females housed in different containers. In contrast, virgin females housed with genitalia-ablated males or group-housed females with antennae ablated did not significantly promote ootheca production compared to isolated females. Daily addition of the primary sex pheromone component to the container did not promote ootheca production in isolated females. Another line of study showed that grouped females make parthenogenesis more sustainable than previously known; a founder colony of 15 virgin females was sufficient to produce female progeny for a period of more than three years.


CONCLUSIONS
Group-housed females promote and stabilize asexual ootheca production compared to isolated females, and that this promotion is triggered by female-specific chemosensory signals (other than sex pheromone) primarily detected by antennae. Promotion of ootheca production between females is likely to be an early stage of social cooperation, reminiscent of the foundation and maintenance of a colony by female pairs in the eusocial termiteReticulitermes speratus.
Graduate School of Life Science, Hokkaido University, Sapporo, 060-0810 Japan.
Ko
K
Katoh
Research Institute for Electronic Science, Hokkaido University, Sapporo, 060-0812 Japan.
Masazumi
M
Iwasaki
Graduate School of Life Science, Hokkaido University, Sapporo, 060-0810 Japan.|Present address: Division of Applied Entomology and Zoology, Graduate School of Agriculture, Tamagawa University, Machida, Tokyo, Japan.
Shouhei
S
Hosono
Research Institute for Electronic Science, Hokkaido University, Sapporo, 060-0812 Japan.
Atsushi
A
Yoritsune
The Institute of Low Temperature Science, Hokkaido University, Sapporo, 060-0819 Japan.
Masanori
M
Ochiai
Faculty of Science, Hokkaido University, Sapporo, 060-0810 Japan.
Makoto
M
Mizunami
Research Institute for Electronic Science, Hokkaido University, Sapporo, 060-0812 Japan.
Hiroshi
H
Nishino
Antenna; Chemosensory signal; Cockroaches; Parthenogenesis, group effect; Periplanone; Sex pheromone; Sexual reproduction