An investigation of patterns of connectivity among populations of the Australian mosquito (Aedes vigilax) using mitochondrial sequences and microsatellite loci

Abstract

The fluctuating changes in climate during the Pleistocene period played an important role in population genetic structure of many species in northern and southern Australia. I investigated the population structure and history of a widespread Ross River Virus vector, the Australian saltmarsh mosquito (Aedes vigilax) across the continent. The main aim of this study was to determine levels of connectivity among sites across the continent. This information is important for disease control, because it can inform about the likely spread of pesticide resistance, as well as the scale at which control measures should be undertaken.

In this study, the genetic structure of 379 individuals of this vector (Ae. vigilax) (Skuse) was examined and a comparative approach was taken in analysing patterns of genetic variation. The methods were mitochondrial COI sequence variation and nuclear variation at three microsatellite loci. Analysis of mitochondrial DNA data detected 119 haplotypes of which more than 50% were unique. Bayesian analysis revealed two distinctly divergent clades. Clade-I was only found in eastern Australia (Queensland and New South Wales), whereas Clade-II was found throughout the sampling area (Northern Territory, Western and eastern Australia). A molecular clock calibration was used to estimate the divergence time between the two clades to be 0.924 million years. The Nested Clade Analysis (NCA) indicated that there was significant geographical structure and suggested divergence between clade-I and clade-II by allopatric fragmentation. Increasing aridity during the Pleistocene was suggested as a factor in structuring Ae. vigilax populations. The Carpentarian barrier in the north was hypothesized to be associated with the divergence between the eastern and western clades. In addition, the Murchison arid barrier in the west could explain the low gene flow between south-western population (Perth site) and other western sites.

A recent demographic expansion in the late Pleistocene (6 000 – 13 000 years ago) was inferred from haplotype mismatch distribution analysis. Fu’s Fs tests also indicated evidence of significant population expansion at most sites.

Nested clade analysis demonstrated a range expansion in the western clade, suggesting movement from west to east and from north-western to south-western Australia; while the range expansion in the eastern clade populations was proposed to be from south-eastern to north-eastern. An analysis of molecular variance (AMOVA) indicated contemporary structure at different geographic scales. Overall the mitochondrial gene showed significant levels of population structuring for the subdivision of populations into eastern and western regions. The analysis indicated that individuals of clade-I showed very little genetic structure, whereas clade-II did have significant structure, mostly explained by samples from south-western Australia. The AMOVA analysis of nuclear genes showed some spatial genetic structuring indicating limited gene flow among populations. Microsatellite data supported the geographic structure between east and west, and a significant isolation by distance effect.

I hypothesise that the two clades may have diverged on opposite sides of the continent during the Pleistocene about one million years ago. More recently there has been subsequent secondary contact caused by clade-II individuals expanding their range from west to east. Clade-II populations also showed expansion from the north-west to the south-west; whereas clade-I expanded but only along the east coast. The overlap of eastern clade haplotypes in the eastern region suggested secondary contact among eastern and western clades in eastern Australia. Based on microsatellite data, there was no evidence that the divergence had led to speciation, tests for Hardy Weinberg Equilibrium, Linkage Disequilibrium and Assignment analyses provided no evidence for reproductive isolation between these two clades. This suggests that the Australian Ae. vigilax is not a cryptic species complex.

In summary, this study demonstrated the importance of contemporary and historical processes in determining the population genetic structure of Australian Ae. vigilax. It suggests that the arid barriers may have had an impact on geographic variation of widespread populations of this species to form east-west fragmentation during the Pleistocene period. The data reveal patterns of recent population expansion of Ae. vigilax populations that in the west expanded from west to east. Then subsequently secondary contact occurs with populations in the east. Studying the ability of Ae. vigilax to disperse is important and may have relevance to control programs to prevent the spread of arboviruses carried by this species.