Graduation Term

2017

Degree Name

Doctor of Philosophy (PhD)

Department

School of Biological Sciences

Committee Chair

Thomas Hammond

Abstract

A fundamental step that occurs during sexual reproduction is meiosis, which is a specialized type of cell division. During meiosis, pairs of chromosomes exchange genetic information via recombination. At this point, the genome is particularly susceptible to viruses and other foreign genetic invasions. Therefore, it is important to protect the genome to prevent the transmission of foreign genetic materials to the offspring. There are several mechanisms work together to protect host genome from foreign genetic materials. These are known as “genome defense mechanisms”.

The fungus Neurospora crassa is one of the best organisms for genome defense studies due to the presence of at least three genome defense mechanisms; including Repeat Induced Point mutation (RIP), Quelling, and Meiotic Silencing by Unpaired DNA (MSUD).The main focus of my dissertation is the MSUD pathway.

MSUD is a process that detects and silences unpaired DNA between homologous chromosomes. During MSUD, Homologous chromosomes are scanned for unpaired regions by unknown protein complexes. These protein complexes may also contribute to homology search required by some DNA repair pathways. Therefore, identification of these proteins could thus have a significant impact for cancer research. Hence, one part of my dissertation is to identify and characterize novel proteins that detect unpaired DNA during meiotic silencing. In my findings, I have found a putative SNF2-family protein (SAD-6) required for efficient MSUD in Neurospora crassa and it is closely related to a protein called Rad54, which involved in the repair of DNA double-strand breaks by homologous recombination.

Moreover, I was able to identify and characterize Neurospora crassa sad-7, a gene encoding a protein with RNA recognition motif (RRM). My experiments have confirmed that SAD-7 in N. crassa, is required for fully-efficient MSUD in the presence of unpaired DNA.

Additionally, I have focused on Meiotic drive elements. These elements are found in eukaryotic genomes. In general, genetic loci are transmitted to the offspring during sexual reproduction by following the Mendelian inheritance patterns. However, there are some selfish loci that are capable of bias their own transmission rates through meiosis or during gametogenesis in the presence of a competing locus. These are known as meiotic drive elements. Neurospora crassa has a meiotic drive element known as Spore killer-2 (Sk-2) and it achieves the biased transmission by spore killing.

When Sk-2 is crossed into a Spore killer sensitive opposite mating type (SkS), hypothetically there should be a mixed offspring population of killer resistant and killer sensitive ascospores. Surprisingly, when analyzing the ascospores, nearly all the survived ascospores express the Sk-2 genotype and all the ascospores with the Spore killer sensitive genotype are non-viable. However, there are a little known about the exact location of Sk-2 meiotic drive element and it’s mechanism of transmission. In my experiments, I was able identify a genetic element located in Neurospora chromosome III that is required and sufficient for spore killing.

Overall, my results provide new insights to the search and unpaired DNA detection during meiosis and also the identification of the genetic element required for spore killing sheds lights towards the understanding of the spore killer mechanism in Neurospora crassa.

Access Type

Dissertation-Open Access

DOI

http://doi.org/10.30707/ETD2017.Ralalage.D

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