Guo Ding:
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Reproductive division of labor is a hallmark of the social insects. The individuals produced by the same mother queens can develop either to the reproductive caste or non-reproductive caste fate. The difference in the developmental process of the gonad during the developmental course of two female castes (queen and worker) thus is the key for understanding the genetic mechanisms underlying the caste determination and differentiation. Pharaoh ant (Monomorium pharaonis) is an excellent model system to study the differentiation process of the ovary development in the female individuals because the queen individuals develop with full function ovaries while the ovary development in worker individuals is suppressed in early developmental stage. However, the detailed developmental process of ovary is still unclear and many molecular experimental tools are still under-developed in this nonmodel organism thus present obstacle for the study on caste developmental regulation in this species.
In my thesis, I aim to establish some molecular experimental tools in M. pharaonis and apply these tools to explore some key genes related with the ovary development in this species. In the first chapter, I reported a set of whole-mount in situ hybridization (ISH) methods for tracing the gene expression patterns in the embryos and larvae of pharaoh ant and applied these methods to test three germline marker genes, nanos, vasa, and oskar, in embryonic stages. I confirmed the expression pattern of the three marker genes in germline of pharaoh ants, but only nanos and oskar show expression in the posterior pole of embryo at the time when the eggs were freshly laid. Thus, the mRNAs of these two genes can be derived from maternal origin while vasa mRNA is transcribed from the zygotic genome.
In the second chapter, I traced the expression of protein and mRNA of vasa, an essential gene for ovarian development, following the developmental process of embryo and larva in pharaoh ants. Workers are likely to lose their germ cells during gastrulation and germ band elongation stages. Dynamic observation of embryonic development and immunostaining of Vasa protein identified two germ cell migration patterns corresponding to reproductive and non-reproductive castes. Additionally, the transcriptome data of two types of embryos revealed abnormal cell migration and sequential programmed cell death are probably the main reasons of germ cell degeneration in workers of M. pharaonis.
In the third chapter, I studied the functions of two developmentally canalized genes associated with castes. I first investigated their functions in M. pharaonis adult ovary development by RNA interference (RNAi) and confirmed both genes play a role in the ovary development like in fly. Then larvae RNAi was conducted to examine their potential function in caste differentiation during development. I found the genes involved in ovary development that might promote the establishment of canalization traits associated with castes in ants. Overall, my PhD program has presented the developmental process of reproductive organ in female castes and explored the potential regulatory mechanism of ovary development in pharaoh ant. This thesis provides useful information about ovary development to facilitate the understanding of evolution of caste differentiation and even the origin of caste system in ants.