Date of Award
Doctor of Philosophy (PhD)
School of Biological Sciences
Meiosis is a fundamental and highly conserved biological program that is required for sexual reproduction in eukaryotes. During meiosis, duplicated chromosomes synapse with their homologous partner and undergo a physical exchange of genetic material through recombination. The meiotic products resulting from this program contain novel combinations of alleles that are inherited by individuals in the succeeding generation. The random assortment of alleles during meiotic recombination allows for the chance that any such allelic combination may provide an increase (or decrease) in fitness for an individual. Alleles, or combinations of alleles, that are beneficial to the organism may be retained within a population through evolutionary pressures, while deleterious alleles, by these same pressures, may be selected against and lost from a population. In this way, a population can become more evolutionarily fit through selection of the most advantageous alleles, which are randomly inherited. Indeed, Mendelian genetics dictates that each unlinked allele in a sexual cross has an equal probability of being inherited in the succeeding (F1) generation. This basic assumption gives each allele a fair opportunity to be represented in the F1 population, where it can then be evaluated for fitness by traditional evolutionary pressures.However, some alleles or loci do not follow typical Mendelian inheritance patterns and are preferentially inherited whenever they are present in a meiotic event. These selfish genetic elements are termed Meiotic Drivers (MDs) or Meiotic Drive Elements (MDEs) and can bias their inheritance ratios in their favor at the expense of a competing allele or locus, leading to a >50% inheritance ratio. Meiotic Drivers can persist or even increase within a population, even if they confer an overall fitness cost to their host because they act in their own interests, rather than altruistically. Due to the fitness costs associated with harboring a Meiotic Drive Element, host genomes have evolved antagonistic mechanisms (suppressors) to limit their spread. The resulting molecular arms race between the host genome and the parasitic Meiotic Driver can contribute significantly to reproductive isolation and promote rapid evolution between populations. Understanding the molecular mechanisms of how Meiotic Drivers bias their transmission ratios can ultimately lead to a better understanding of genetic conflict and how it can shape the evolution of a genome. In this dissertation, I will present research that identifies and characterizes two Meiotic Drive Elements found within the genome of the filamentous fungus, Neurospora crassa and will discuss the molecular and evolutionary implications of meiotic drive in this fungus. Fungal meiotic drive most often manifests in spore killing, where spores (fungal meiotic products) that do not inherit the Meiotic Driver (Spore Killer) are eliminated by a ‘toxin’, while spores that do inherit the Meiotic Driver survive because they produce an ‘antidote’. Thus, surviving spores from a heterozygous cross (a cross between a Spore Killer and a non-Spore Killer) almost always inherit the Spore Killer. In Chapter I, I present a summary of spore killing meiotic drive elements in Neurospora and their associated molecular mechanisms. In Chapter II, I identify the gene responsible for spore killing in N. crassa Spore Killer-2 (Sk-2). This gene, rfk-1, is located on the right border of the Sk-2 locus, which allows it to escape detection of Meiotic Silencing of Unpaired DNA (MSUD), an RNAi-mechanism that can suppress Spore Killers. I show that the rfk-1 transcript contains four exons, three introns, and a premature stop codon that undergoes A-to-I mRNA editing exclusively during meiosis to produce a 130-amino acid product that is not produced during vegetative growth. Finally, I present evidence that rfk-1 originated from a partial gene duplication event of ncu07086, which contains a putative AtpF superfamily domain. In Chapter III, I expand upon the previous chapter and identify a functional role for A-to-I mRNA editing in spore killing. Specifically, I show that RNA editing is required for rfk-1 to produce an mRNA transcript that can produce a toxic version of the RFK-1 protein (RFK-1B). In the absence of editing, RFK-1B cannot be produced and spore killing does not occur. Expression of rfk-1B in vegetative tissue is usually prevented because mRNA editing is not known to occur in vegetative cells of Ascomycete fungi. I show that expression of transgenic rfk-1B, but not unedited rfk-1A, in vegetative tissue is toxic to the fungus, suggesting that mRNA editing protects the host organism from the toxic effects of RFK-1B. Finally, we show that co-expression of the resistance gene, rskSk-2, in vegetative tissue can rescue the toxic effects of rfk-1B. In Chapter IV, I identify a mutation that blocks Sk-3-based spore killing and name this mutation rfk-2UV. The rfk-2 mutation was purified through a series of back-crosses. Whole genome sequencing and three-point cross analysis was used to map mutation to a region 15.6cM from mus-52 on Chromosome III. I show that a knockout of this locus prevents spore killing. Interestingly, cloning this locus into a sensitive host is not sufficient for killing, unlike the AH36 locus identified in Chapter II. This chapter provides the molecular groundwork for identification of the killing element of Sk-3, which will in turn help identify the evolutionary trajectories that led to Sk-2 and Sk-3 spore killers.
Rhoades, Nicholas, "Investigating the Molecular Mechanisms of Spore Killing by Neurospora’s Meiotic Drive Elements" (2023). Theses and Dissertations. 1772.