 Abstract | This research investigates the evolution of Neotrigonia species (Bivalvia: Palaeoheterodonta), the remaining extant genus of the Trigonioida, a group of bivalves endemic to Australian waters. The intent of this research was to review the current systematics, investigate phylogeny and phylogeography of the genus, and advance scientific knowledge in regard to the presence of doubly uniparental inheritance in Neotrigonia, as well as to address some aspects of reproductive strategy and outline the process of oogenesis. The research has resulted in a thesis in manuscript format, where Chapter 1 is a general introduction to the thesis as a whole, Chapters 2-5 inclusive are research manuscripts, and Chapter 6 is a general discussion of the completed research.
In chapter 2, the type material of all of the seven extant, nominal species of Neotrigonia Cossman 1912 are reviewed and illustrated, based on available museum specimens and fresh collections. The type localities and currently-known distributions for each extant species are included. A cladistic analysis was performed using morphological characters of Neotrigonia species living and fossil, using Eotrigonia subundulata and Trigonia miriana as an outgroup. Results from parsimony analysis show that all Neotrigonia form a monophyletic clade, in which living and fossil Neotrigonia form reciprocally monophyletic sub-groups. The species status of Neotrigonia bednalli, Verco 1907, is revised based on examination of all available types, museum specimens and a relatively large number of newly-collected specimens from southern Australian waters. This assessment suggests that N. bednalli is a junior synonym of N. margaritacea. Species status is accepted for N. gemma, N. lamarckii, N. uniophora, N. strangei and N. kaiyomaruae. However, reclassification of N. strangei specimens from Western Australia to N. margaritacea would revise the previously disjunct distribution of this species to a narrower range in NSW. This chapter demonstrates the limitations in relying on shell morphology only for species classification in the Neotrigonia.
The contemporary knowledge of ocean currents, temperatures, and geological and climatic history across southern Australian waters represents a useful framework for phylogeographical analyses. There are already a number of studies that show coincident distribution patterns within some marine invertebrate groups across the Maugean, Flindersian and Peronian marine provinces. In Chapter 3, I examine the genetic structure of Neotrigonia margaritacea and Neotrigonia lamarckii. Phylogenetic analyses based on COI and ITS gene sequence data reveals a split between southern Australian Neotrigonia margaritacea and eastern Queensland Neotrigonia lamarckii. The molecular analyses confirmed my synonymisation of N. bednalli to N. margaritacea. Population genetic analyses of the Neotrigonia margaritacea COI gene, in four different populations located hundreds of kilometres apart, revealed insight into genealogical pathways amongst haplotypes. These networks showed that there was no shared haplotypes among populations and most populations were significantly far from panmixia. The highest haplotype diversity was recorded from the Port Lincoln (South Australia) population. Haplotype variations across the range are discussed in terms of estimated population sizes and geographical barriers.
Several species of bivalves have been reported to have two mitochondrial DNA types, maternal and paternal. This system of mtDNA inheritance is known as doubly uniparental inheritance (DUI). In Chapter 4, the presence of the DUI phenomenon in Neotrigonia margaritacea is investigated within a phylogenetic framework for Paleoheterodonta, using COI and 16S rDNA molecular data. Results indicate the presence of DUI in Neotrigonia margaritacea and provide evidence for a masculinization event within this taxon. This phenomenon has so far been identified in six superfamilies of bivalves, so the new record of DUI in N. margaritacea was incorporated into a phylogenetic tree addressing the question of a single or multiple origins of DUI in Bivalvia. Parsimony transformations indicate that DUI is likely to be the ancestral state for all Bivalvia.
In Chapter 5, the ultrastructural stages of female gametogenesis are described for Neotrigonia margaritacea. The morphology of oocytes and gonad tissue are described for the first time using electron microscopy and histology techniques. Throughout the summer period, the ovary contains oocytes in various developmental stages. Oocytes develop from oogonia derived from protogonia and then undergo three distinct stages of oogenesis: previtellogenesis; vitellogenesis; and postvitellogenesis (or presence of mature oocytes). Based on gonad tissue and oocyte morphology, and as well as laboratory observations, it is inferred that Neotrigonia margaritacea is sequentially tachitictic, thus a trickle (continuous) spawner over an extended summer season.
In conclusion the museum collections of Neotrigonia and current systematics have provided valuable information on classification and distribution of this relic bivalve genus. Morphological analysis has enabled preliminary synonymisation of species to establish species distributions. The results from molecular data confirmed aspects of phylogeny and revealed phylogeographic structure of Neotrigonia margaritacea in Southern Australian waters. The new molecular information regarding the presence of DUI and novel insight into reproductive strategies further our understanding of the evolutionary affinities of Neotrigonia. Based on the integration of these multidisciplinary results conservation assessment is suggested for Neotrigonia margaritacea.
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