Biological Sciences, School of


Date of this Version

Spring 5-19-2010


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Biological Sciences, Under the Supervision of Professor Steven D. Harris. Lincoln, Nebraska: August, 2010.
Copyright 2010 Haoyu Si


One of the distinguishing features of fungal cells is their highly polarized model of growth. Both yeast cells and hyphal cells grow by cell surface expansion at specified cortical sites. Although the same general mechanisms are likely to be involved in controlling the establishment of hyphal polarity in budding yeast and filamentous fungi, it is noticeable that hyphal cells are organized in a fundamentally different manner to yeast dells. For example, hyphal cells organize formins, septins and actins at the division site while simultaneously retain the same machinery at the tip; whereas yeast cells undergo a transient period of isotropic growth with mitosis and cell cycles. Among filamentous fungi, Aspergillus nidulans had been proven to be a particularly valuable model. The genetic tractability of this fungus coupled with the availability of sophisticated post-genomics resources has enabled the identification and characterization of numerous genes involved in hyphal morphogenesis. One objective of this study was to determine the extent to which components of the S. cerevisiae bud site selection module were conserved in filamentous fungi. We have identified and examined the function of bud site selection homologues of Bud3 (AN0113.3), Bud4 (AN6150.3), and Axl2 (AN1359.3) in A. nidulans, even though the sequence conservation is largely limited to domains that are presumed to be functionally important (i.e., the GEF domain of Bud3, and the anillin-like and PH domains of Bud4). We also identified homologues of Msb2 (An4701.3) and Rga1 (An1025.3), which are the small GTPase Cdc42 related proteins. In this article, their unique functions for hyphal morphogenesis were characterized towards understanding the function of these genes and the mechanisms involved in polarized hyphal growth, septation and secondary developments in A. nidulans. I also highlight important areas for future investigation.