In insects, there are two different ovarian types: panoistic and meroistic, depending of the structure of the ovary and the type of cells that form the ovarian follicle. The panoistic ovaries are the most basal model, and in them the ovarian follicle is formed exclusively by one germinal cell (the oocyte) surrounded by a monolayer of follicular cells. In meroistic ovaries, thoroughly studied in Drosophila melanogaster, the ovarian follicle is formed by one oocyte accompanied by a series of nurse cells, and a monolayer of follicular cells surrounding them (Büning, 1994).
However, regardless of the ovary type, the common final function is to obtain a correct embryo development. To reach this final goal, a series of genes should be transcribed and translated on-site in the ovarian follicle at the correct time. Some of these genes are conserved in structure and function in panoistic and meroistic ovaries, whereas other genes are specific of one of these types. A third group of genes have the same structure, but exert different functions in the two ovarian types.
The long-term objective of our group is to know how the transition from the panoistic to the merositic type occurred in evolution. To develop our objective we will use the cockroach Blattella germanica as experimental model. This cockroach has been the insect model in our group for some 30 years, and this circumstance provides a thorough source of information about the biology, physiology and endocrinology of this cockroach. During the last years, three ovarian transcriptomes from different stages of female development have been sequenced and characterized (Irles et al., 2009). At the same time, the miRNA presents in the ovary of this species have been sequenced and identified (Cristino et al., 2011), and the expression of few of them studied (Tanaka and Piulachs, 2012). We also take advantage that this insect species is very sensitive to the RNAi methodologies, allowing us to study gene function by depleting mRNA levels of the chosen gene (Ciudad et al., 2006).
At present we are developing to main projects:
1. Determining oocyte polarization.
The localization of mRNA and proteins into the oocyte, early in oogenesis, will be of key importance later for the correct embryo development. Usually, the germinal band localization in the egg is determined by the position that the nucleus occupies in the mature oocyte. Oocyte polarization has been well studied in Drosophila melanogaster, a species with meroistic ovaries, where the oocyte nucleus is localized in a dorsal-anterior position. We have selected a few genes described in D. melanogaster that were involved in this process (like capicua, oskar, torso-like and EGFR) to study their expression, localization and function in the panoistic ovary of Blattella germanica, in which the oocyte nucleus localizes in a ventral-central position. We expect to understand how genes that are generally involved in oocyte polarity and are conserved in structure, interact each other or establish new connections in the transition from the panoistic to the meroistic model.
2. Regulation of cell proliferation during oogenesis
Control of cell proliferation is very important to keep the size and form of different organs, and a number of proteins, mainly belonging to the Hippo pathway, are fundamental in this control. In relation to oogenesis, is important to know how follicular cells proliferate and how they avoid over-proliferation, which could lead to of ovarian follicle death. In the panoistic ovary of B. germanica we used RNA interference (RNAi) to study different signaling pathways (Hippo, Notch, EGFR, MAPK…) in order to understand their function in relation to oocyte maturation. Also, and most intriguing, we study the possible role of microRNAs (miRNAs) a a new regulatory layer ofthese pathways. The miRNAs of Blattella germanica have been recently described, and the idea is to identify miRNA that can modulate the expression of the main components of the above pathways in our model species. The information generated would serve as a baseline for comparisons with species possessing meroistic ovaries.