| The Microsporidia are a lineage of minute, single-celled, intra-cellular parasites that lack mitochondria, and that have been considered to be among the most primitive eukaryotic organisms. Although small subunit ribosomal RNA (SSU rRNA) sequences and cytological data appeared to imply that Microsporidia are indeed an ancient lineage, that contention has been contradicted by data from several protein-coding genes, which support instead a relationship with true Fungi (Eumycota).; I attempted to determine, through analysis of ribosomal RNA, whether Microsporidia are more likely to be an ancient lineage or to be related to Fungi. Subsequently, using a nuclear protein-coding gene and SSU-rRNA, I investigated whether Microsporidia originated within the Fungi, and whether any extant Fungi are close relatives of Microsporidia.; My initial results showed that the SSU-rRNA data support a fungal origin equally as well as an ancient origin, when maximum likelihood methods incorporating site-to-site rate variation are employed, and highly-divergent sequences are excluded.; In attempting to identify the fungal lineage most closely related to Microsporidia, I sequenced 70-kD heat-shock protein genes from a broad sampling of Fungi, and analyzed them with known microsporidian sequences. The microsporidian HSP70 sequences could not be placed within the Fungi; they are highly divergent and perhaps paralogous.; Finally, my likelihood-based analyses of a large data set of fungal SSU-rRNAs suggest that Microsporidia arose within a fungal clade encompassing the Blastocladiales (Chytridiomycota) and the insect-pathogenic Entomophthorales (Zygomycota). It is ironic that, although rRNA sequences helped to establish the incorrect hypothesis of an ancient origin, I have now used them to infer a relationship between Microsporidia and a specific fungal lineage.; In additional work, I investigated the phylogenetic distribution of a putatively ancient intron in the triosephosphate isomerase (TPI) genes of mosquitoes. This particular intron served as the keystone of one phase of the debate on the origin of introns and their postulated role in primordial gene assembly. I collected sequence data for the TPI gene from a diverse set of mosquitoes, other Diptera, and other insects. The intron was present only in a restricted lineage leading to mosquitoes and their nearest relatives, the chaoborid midges. This indicates that the intron is most probably a recent insertion in this lineage, and is unlikely to have played a role in exon shuffling in the common ancestor of all life. |
