The gene encoding the insulin-like androgenic gland hormone in an all-female parthenogenetic crayfish

Male sexual differentiation in crustaceans is controlled by the androgenic gland (AG), a unique male endocrine organ that, in decapods, is located at the base of the 5th pereiopod. In these animals, the insulin-like androgenic gland hormone (IAG) is the major factor secreted from the AG to induce masculinization and maintain male characteristics. It has, however, recently been proposed that this hormone also plays a role in growth and ovarian development in females. In this study, we tested such a possibility by searching for the IAG gene in the marbled crayfish, a parthenogenetic animal that reproduces asexually to form an all-female genetic clone. Based on the phylogenetic relationship between the marbled crayfish and Procambarus fallax, a gonochoristic species of the same North American Cambaridae family, we searched for the IAG gene in the marbled crayfish and then fully sequenced it. The open reading frame of the gene was found to be completely identical in the two species, and their introns shared over 94% identity. It was also found that, in addition to its expression at the base of the 5th pereiopod and in the testes of male P. fallax crayfish, IAG was expressed in the muscle tissue of P. fallax males and females and even of the parthenogenetic marbled crayfish. These findings provide new insight into possible functions of IAG, in addition to its role as a masculinization-inducing factor, and also constitute the basis for a discussion of the evolutionary relationship between the above two species.

十足目甲壳动物的雄性性别分化由雄性腺（androgenic gland, AG）调控，该器官为雄性特有内分泌器官，在十足类动物中位于第5步足基部。在这类动物体内，胰岛素样雄性腺激素（insulin-like androgenic gland hormone, IAG）是雄性腺分泌的核心效应因子，可诱导雄性化进程并维持雄性个体特征。不过近期已有研究提出，该激素在雌性个体的生长与卵巢发育过程中也发挥着一定作用。本研究以大理石纹螯虾为研究对象，通过搜寻其体内的IAG基因验证这一假说：大理石纹螯虾为孤雌生殖物种，可通过无性繁殖产生全雌性遗传克隆群。基于大理石纹螯虾与同属北美螯虾科（Cambaridae）的雌雄异体物种佛罗里达螯虾（Procambarus fallax）的系统发育关系，我们对大理石纹螯虾的IAG基因进行了搜寻并完成全长测序。结果显示，二者的基因开放阅读框（open reading frame）完全一致，内含子序列同源性亦超过94%。研究还发现，除在雄性佛罗里达螯虾的第5步足基部与精巢中表达外，IAG在雄性与雌性佛罗里达螯虾的肌肉组织，乃至孤雌生殖的大理石纹螯虾体内均有表达。上述发现不仅为IAG除诱导雄性化之外的潜在功能提供了新的研究视角，也为探讨上述两个物种间的演化关系奠定了基础。