I studied the mt genomics of hemipteroid insects with a comparative approach. My aims were, first, to get a better understanding of the evolution of mt genomes of insects and, second, to explore the potential of mt gene rearrangements as phylogenetic markers for hemipteroid insects. Five questions were addressed:
(1) Do rates of gene rearrangement in the mt genomes of insects vary?
(2) Are the rates of mt gene rearrangement and mt nucleotide substitution correlated?
(3) What factors affect the rate of mt gene rearrangement?
(4) Are there any rearrangements of mt genes which are phylogenetically informative for hemipteroid insects?
(5) What are the possible mechanisms of mt gene rearrangement in hemipteroid insects?
To achieve the aims and address the questions above, I sequenced part
of the mt genomes of seven species of hemipteroid insects plus the entire mt genomes of three species from three of the four orders of hemipteroid insects: the wallaby louse, Heterodoxus macropus (order Phthiraptera), a lepidopsocid species (Psocoptera) and the plague thrips, Thrips imaginis (Thysanoptera). I retrieved data on mt gene arrangements for more than 400 other species of insects from GenBank. I also retrieved the entire mtDNA sequences of the 17 insects which were available in GenBank. I then compared the mt gene arrangements of these hemipteroid insects with one another, with those of other insects and with the hypothetical ancestral arrangement of insects. I also compared the rates of mt gene rearrangement and the rates of mt nucleotide substitution of the 20 insects whose mt genomes have been sequenced entirely (three genomes from my studies and 17 genomes from GenBank).
There are seven major findings from my work:
(1) Most of the mt genes, including the protein-coding genes and rRNA genes, in H. macropus and T. imaginis, have rearranged (Chapters 3 and 5).
(2) All species studied from three of the four orders of hemipteroid insects (Phthiraptera, Psocoptera and Thysanoptera) have mt genomes with rearranged protein-coding genes and tRNA genes (Chapter 4).
(3) The mt genome of T. imaginis has both duplicate control regions and rearrangements of most of the mt genes (Chapter 5).
(4) The two mt rRNA genes are distant from one another and from the putative control region in T. imaginis (Chapter 5).
(5) Two novel gene boundaries in species from two closely-related orders of insects, T. imaginis (Thysanoptera) and a lepidopsocid sp. (Psocoptera), evolved by evolutionary
convergence (Chapter 5).
(6) The rate of rearrangement of mt genes varies substantially among the four orders of hemipteroid insects and this rate is much higher in two of the four orders of hemipteroid insects, the Phthiraptera and the Thysanoptera, than in other orders of insects studied (Chapter 6).
(7) The rate of gene rearrangement and the rate of nucleotide substitution are correlated significantly in the mt genomes of insects (Chapter 6).
I conclude that:
(1) Duplicate control regions may facilitate tandem duplication and deletion of genes in mt genomes (Chapter 5).
(2) The mechanism of rRNA gene transcription in the mt genome of T. imaginis may be novel for an arthropod (Chapter 5).
(3) Single gene boundaries may evolve by convergence and thus are not reliable phylogenetic markers for hemipteroid insects
(Chapter 5). However, there are many novel contiguous boundaries of three or more genes in the mt genomes of hemipteroid insects which are less likely to evolve by convergence and thus are likely to be reliable phylogenetic markers (Chapters 3-6).
(4) A high rate of nucleotide substitution may lead to a high rate of gene rearrangement in the mt genomes of insects (Chapter 6).
(5) Hemipteroid insects are a good model system for studies of the evolution of animal mt genomes (Chapter 3-6).