Differentiation of the reproductive tract between dominant and subordinate workers in the Japanese queenless ant Diacamma sp

Ovarian development and insulin-signaling pathways during reproductive differentiation in the queenless ponerine ant Diacamma sp

In many social hymenopteran species, workers possess functional ovaries that are physiologically inactive in the presence of queens. We investigated the ovarian regulatory mechanism of workers and reproductives in a queenless ponerine ant, Diacamma sp., using histological and molecular techniques. In this ant, clear reproductive differentiation occurs via a highly sophisticated dominance behavioral interaction called "gemmae mutilation". This clear and rapid bifurcation of reproductive physiology allows us to elucidate the detailed ovarian differentiation process. Histological characteristics of functional ovaries (fusomes and ring canals) were found in both workers and reproductives, suggesting that early oogenesis is not blocked in workers. Since insulin/insulin-like growth factor signaling (IIS) is known to control insect reproduction, orthologs of 2 positive IIS regulators, insulin receptor and serine-threonine kinase Akt (protein kinase B), were cloned in Diacamma (DiaInR, DiaAkt); their expression patterns during reproductive differentiation were examined by real-time quantitative polymerase chain reaction; DiaInR and DiaAkt were strongly expressed in the gasters of reproductives. Whole-mount in situ hybridization of ovaries indicated that DiaInR and DiaAkt were expressed in nurse cells, oocytes, and upper germarial regions of reproductives but not of workers. Our data suggest that the IIS pathway accounts for reproductive differentiation in late oogenesis.

Testing homology with morphology, development and gene expression: sex-specific thoracic appendages of the ant Diacamma

Females of the ants belonging to the queenless genus Diacamma have a pair of unique tiny thoracic appendages, called "gemmae," located on the mesothoracic segment. They are covered with sensory hairs, filled with exocrine glands and are involved in the behavioral regulation of reproduction. We report here a morphological, developmental, and genetic study of the development of the gemmae. Both male and female larvae have dorsal mesothoracic discs, although differing in shape and fate. In Diacamma ceylonense, we show that, contrary to butterflies, these discs specify parts of the adult thorax in addition to wing tissues, as in Drosophila. We have cloned and studied the expression of wingless (wg) and scalloped (sd), two genes known to play a critical role in wing morphogenesis in Drosophila. In the fly's mesothoracic dorsal disc, sd is specifically expressed in the wing pouch. In Diacamma, we show that sd is also expressed in male dorsal thoracic discs, whereas its expression was undetectable in females. From this result and observations of shape and growth of cultured isolated discs, we suggest that gemmae originate from a more ventral part of the dorsal disc than the wing pouch and discuss the pro and cons of gemma/wing homology.

Queen-worker differences in spermatheca reservoir of phylogenetically basal ants

Ant queens mate when young and store sperm in their spermatheca to fertilize eggs for several years until their death. In contrast, workers in most species never mate. We have compared the histological organization of spermathecae in 25 poneromorph species exhibiting various degrees of queen-worker dimorphism. The spermathecae of both castes in all species are similar in having a reservoir connected by a sperm duct to the ovary, and a paired gland opening into this duct. The reservoir of queens typically has a columnar epithelium in the hilar region (near the opening of the sperm duct), whereas the epithelium in the distal region is cuboidal. Abundant mitochondria together with apical microvilli and basal invaginations indicate an osmoregulatory function. In contrast, the reservoir epithelium of workers is flattened throughout and lacks these transport characteristics. This single difference shows the importance of a columnar epithelium in the reservoir for sperm storage. However, our data have not revealed inter-specific variations in the development of the hilar region linked with higher fecundity. We have found no consistent differences in associated structures, such as the spermatheca gland or sperm ducts, or in the musculature between queens and workers.