Conilurine Rodent Evolution: The role of ecology in modifying evolutionary consequences of environmental change

Ford, Frederick David (2003) Conilurine Rodent Evolution: The role of ecology in modifying evolutionary consequences of environmental change. PhD thesis, James Cook University.

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Abstract

Conilurine rodents are the most speciose and ecologically diverse rodent group within Australia. Previous research has demonstrated that conilurines are members of a larger Australasian Murid tribe, the Hydromyini, and are generally thought to be an endemic Australian radiation with a fossil history dating to the mid Pliocene. However, monophyly of conilurines is not well established due to the distant placement of some genera in recent systematic studies. I investigated the evolution of conilurine rodents using a combination of molecular and ecological techniques. I examined phylogenetic relationships among all Australian rodent genera using the mitochondrial control region and a nuclear intron (DHFR 1). These results suggested that conilurines are monophyletic, and that unresolved generic relationships within the group are the product of an early savannah radiation. Building on the results of systematic studies, I examined patterns of speciation and population divergence in model species and species groups. Bioclimatic range modelling and some ecological studies were performed to aid interpretation of molecular evolution within these groups. The overall goal of this thesis is to provide a detailed account of some evolutionary processes affecting an interesting group for which such information is lacking, and to highlight the potential of conilurines for evolutionary study in an Australian context. A more specific goal is to investigate the effect that species ecology has on altering the evolutionary response of different species to the same environmental stimuli. Molecular studies identified a number of taxonomic issues within the group. The genera Notomys, Mastacomys and Pseudomys formed a monophyletic group identified by previous studies. Mastacomys was placed within Pseudomys, with which it was recently synonymised, and Notomys was not clearly separable from Pseudomys. Both Notomys and Mastacomys should be recognised as genera. As a result, Pseudomys was split into several smaller genera. These included resurrection of the genus Thetomys for P. gracilicaudatus and P. nanus, and creation of a new genus Phorolithomys for the recently described pebble-mound mice. A further three groups are apparent within Pseudomys, but splitting of these at the generic level is contingent on further resolving the placement of several species among them. Some individuals previously described as P. delicatulus from north-western Australia appear to be a morphologically cryptic species, and were placed outside the P. delicatulus/ novaehollandiae group in phylogenetic trees. The subspecies P. delicatulus mimulus has no genetic basis. Pseudomys laborifex is probably a subspecies of P. johnsoni, which is an unexpected result given the distribution of P. laborifex in the Kimberley, a region anticipated to contain the sib-taxon to the Top End P. calabyi. Phylogeographic studies of variation in the mitochondrial control region revealed significant differences in the evolution of species groups as result of their differing ecologies. This contrast centres on the morphologically similar pebble-mound mice and delicate mouse group. These are the largest species groups within the current genus Pseudomys. Pebble-mound mice are characterised by the fact that they require pebbly habitat for breeding, and have therefore speciated in allopatry due to erosion and fragmentation of that habitat. Climatic change has also affected pebble-mound mice, and has driven recurrent extinction processes, in part re-enforced by the inability of pebble-mound mice to track climatic change across pebble-free habitats. The delicate mouse group have distributions primarily determined by climate. This means they are periodically isolated and bottlenecked, but also experience secondary contact when conditions permit range expansion. A major finding was that there is no taxon corresponding to Pseudomys pilligaensis, and that individuals of both P. delicatulus and P. novaehollandiae have been included within that species. The genetic distinctness of P. novaehollandiae from P. delicatulus was no greater than that among individuals of P. delicatulus. Whether P. delicatulus and P. novaehollandiae represent sib-species or subspecies was not resolved, but evidence of clinal morphology and probable interbreeding suggests that they have not yet speciated. Pseudomys hermannsburgensis, an arid zone member of the delicate mice, showed little genetic variability, and no geographical structuring across a very large distribution. This result is consistent with ecological studies that show the species possesses morphological adaptations to aridity and exists in relatively mobile populations. A similar result was found for the co-distributed P. desertor. Patterns of genetic structuring are not predictable based on crude habitat preference. It was thought a priori that Zyzomys argurus was a taxon with a similar habitat preference to pebble-mound mice and that it would also exhibit deep genetic structuring between allopatric populations found in rocky habitats across northern Australia. Previous isozyme studies did not find such a relationship, and this was confirmed by mtDNA sequencing. Unlike most other study species, there was no signal from the last glacial maximum, despite the fact that Z. argurus would not have been able to track climate change as many species did. A broad climatic tolerance and good dispersal ability seem to explain the lack of population structuring. Amplification of nuclear copies of the mitochondrial control region hampered efforts to study the chestnut mice, P. gracilicaudatus and P. nanus. However, three clean control region sequences were obtained that apparently confirmed currently recognised species boundaries. Nuclear copies were also amplified from the pebble-mound mice, but primers were designed that prevented their hindrance of phylogeographic studies. The characterisation of evolution among conilurine rodents presented here reveals that species ecology clearly accounts for many differences in patterns of population divergence and speciation. I suggest that ecological interpretation of evolutionary pattern can be extended to a general contrast between the entire conilurine assemblage and marsupials.

Item ID: 77
Item Type: Thesis (PhD)
Keywords: Conilurine rodents, Hydromyini, Phylogenetic relationships
Date Deposited: 03 Aug 2006
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 50%
06 BIOLOGICAL SCIENCES > 0604 Genetics > 060411 Population, Ecological and Evolutionary Genetics @ 50%
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