Characterizing Oocyte Maturation in Nematodes

One major question in biology is how unspecialized stem cells differentiate into specialized cells. For example, embryonic stem cells can develop into entire organisms and stem cells found in bone marrow can develop into red and white blood cells. Stem cells can also give rise to reproductive cells known as gametes, a process known as gametogenesis. One way to study this process is by using model organisms. Nematodes are an advantageous model organism because their germ cells are linearly arranged in a developmental sequence. In other words, we can see the entire process of specialization at one time. In the best-studied nematode, Caenorhabditis elegans, unspecialized stem cells are present at the distal portion of the germline. These cells undergo mitosis and then subsequently enter meiosis (Corsi et al., 2005).

Gametogenesis is composed of two processes: oogenesis, which results in eggs, and spermatogenesis, which results in sperm (Pazdernik et al., 2013). My research project will work to further characterize the process of oogenesis, particularly oocyte maturation. Oocyte maturation is the process by which immature oocytes prepare for fertilization by sperm. In the well-studied nematode worm, C. elegans, oocyte maturation occurs at the end of the first stage of meiosis, known as meiotic prophase I, and is dependent on the presence of sperm. If no sperm is present, oocyte maturation does not occur (Jud et al., 2007). While many experiments have been performed to study oocyte maturation in C. elegans, this process is not as well understood in other nematode species. A related nematode species, Rhabditis sp. SB347, differs from C. elegans in that is has fewer germline cells and exists as self-fertilizing hermaphrodites, males, and females, as opposed to just hermaphrodites and males in C. elegans. As a result, we are interested in studying the differences in oocyte maturation between the two species to explore the diversity of the Nematoda phylum.

This project originated from distinct findings of previous experiments from this lab. Previously, we used a protein known as FBF to understand its role as a regulator of differentiation. We know in hermaphrodite C. elegans, the primary function of FBF is to keep germline stem cells in an undifferentiated, mitotic state (Corsi et al., 2005). These stems cells are fated to undergo meiosis to produce sperm and eggs. The primary vehicles of cell differentiation are proteins, which are produced from a cell’s DNA via an intermediate molecule known as messenger RNA (mRNA). mRNA’s role is to convert the information of DNA into a more readable form to produce proteins. FBF prevents differentiation by inhibiting mRNA activity (Corsi et al., 2005). Thus, based on studies in C. elegans, we expected FBF staining in the distal, mitotically dividing region of the gonad because the FBF protein blocks mRNA function. As expected, the distal region of the gonad stained positively for FBF (Lin et al. unpublished studies). However, this experiment also yielded unexpected FBF staining in the late maturing oocytes of R. sp. SB347, a region of the gonad where oocytes are fully mature and ready to divide. This suggests that FBF may have an unexpected function in oocyte maturation in R. sp. SB347. Building off this finding, we will perform a comparative study of maturing oocytes in both R. sp. SB347 and C. elegans.

Ultimately, this research will provide more insights on the specialization of cells, particularly the specialization of unspecialized germline stem cells into oocytes. This will allow us to further understand differential reproductive strategies across the Nematoda phylum and help us to understand how these strategies evolved.

References:

 

Corsi, Ann K., et al. (2005). A Transparent window into biology: A primer on Caenorhabditis elegans. Worm Book ed. The C. elegans Research Community.

Jud, M., Razelun, J., Bickel, J., Czerwinski, M., & Schisa, J. A. (2007). Conservation of large foci formation in arrested oocytes of Caenorhabditis nematodes. Development genes and evolution, 217(3), 221-226.

Pazdernik, N., & Schedl, T. (2013). Introduction to germ cell development in Caenorhabditis elegans. In Germ cell development in C. elegans (pp. 1-16). Springer New York.

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