The population genetic structure of species may be determined by complex interactions
among many ecological, evolutionary and genetic processes. I investigated the
population genetic structure of coral reef fishes on the Great Barrier Reef (GBR),
Australia to better understand how these various processes may interact in a natural
system. I firstly examined the spatial genetic structure of a low dispersal species to
determine if its genetic structure varied among spatial scales and among regions located
in the centre and on the periphery of its distributional range. I then examined the
population genetic structure of species with different dispersal potentials and among
species sampled at central and peripheral locations in their species range.
Using mtDNA control region sequences and three microsatellite loci, I examined
the spatial genetic structure of a direct developing coral reef fish, Acanthochromis
polyacanthus, with comparatively low dispersal rates. The spatial genetic structure of
this species was scale-dependent with evidence of isolation-by-distance among regions,
but not within regions. Very strong genetic structure was detected among reefs within
regions consistent with a metapopulation model. Pairwise genetic distances increased
from offshore and older populations, to inshore and younger ones, supporting a
metapopulation propagule-pool model of colonisation. Genetic diversities, mismatch,
and coalescence analyses all identified large variation in the demographic history of this
species among populations and regions. Evidence of genetic bottlenecks was detected
by mismatch analysis in the majority of populations sampled, but in most populations
these bottlenecks appeared to be older since genetic diversities and coalescence based
population growth estimates did not indicate recent genetic bottlenecks. In contrast,
three populations displayed low genetic diversities and large population growth rates
indicating a more recent genetic bottleneck. Reductions in genetic diversities of local
populations resulted in overall lower genetic diversity and a higher regional expansion
rate in the southern region located towards the distributional margin of this species. In
all, these results suggest that A. polyacanthus exists as a metapopulation within regions
on the GBR and that metapopulation dynamics may differ among regions located in the
centre and on the periphery of this species.
The pelagic larval duration (PLD) can both affect and record the ecology and
evolution of coral reef fishes and emerging evidence suggests that this trait displays
considerable intraspecific variation. Here I present new estimates of PLD for ten species
of Pomacentridae and two species of Gobiidae, and coupled with previously published
estimates, examine spatial and temporal variation of PLDs within and among these
species. In eight of the twelve species examined here, within-population mean PLDs
differed between sampling times, locations within regions, and among regions. In
contrast, the range of these same PLD estimates overlapped at all spatial and temporal
scales examined in eleven of the twelve species, but not between regions in one species
(Amphiprion melanopus). Therefore, despite tight error estimates typically associated
with estimates of PLD taken from a particular population at a particular time in some
taxa, the overlapping ranges in PLD reported here indicate that the length of the pelagic
larval phase is a much more plastic trait than previously appreciated.
Pelagic larval duration (PLD) is a commonly used proxy for dispersal potential
in coral reef fishes. Here I examine the relationship between PLD, genetic structure and
genetic variability in coral reef fishes from one family (Pomacentridae) that differ in
mean larval duration by more than a month. Genetic structure was estimated in eight
species using a mitochondrial molecular marker (control region) and in a sub-set of five
species using nuclear molecular markers (ISSRs). Estimates of genetic differentiation
were similar among species with pelagic larvae, but differed between molecular
markers. The mtDNA indicated no structure while the ISSR indicated some structure
between the sampling locations. I detected a relationship between PLD and genetic
structure using both markers. These relationships, however, were caused by a single
species, Acanthochromis polyacanthus, which differs from all the other species
examined here in lacking a larval phase. With this species excluded, there was no
relationship between PLD and genetic structure using either marker. Genetic diversities
were generally high in all species and did not differ significantly among species and
locations. Nucleotide diversity and total heterozygosity were negatively related to
maximum PLD, but again, these relationships were caused by A. polyacanthus and
disappeared when this species was excluded from these analyses. These genetic patterns
are consistent with moderate gene flow among well-connected locations and indicate
that at this phylogenetic level (i.e., within family) the duration of the pelagic larval
phase is not the primary factor affecting patterns of genetic differentiation.
Using mtDNA (control region) and nuclear (ISSR) markers, I investigated the
population genetic structure of three congeneric species pairs of pomacentrid coral reef
fishes (Pomacentridae) in the context of species’ borders theory. This theory predicts that population located on the periphery of the species’ range should be smaller and
more fragmented and hence, display stronger genetic structure and lower genetic
diversities compared to more centrally located populations. Each species pair consisted
of one species sampled at two central locations within its geographic range, and another
species sampled at the same locations but which constituted one location toward the
centre of its range and another close to its edge. Contrary to expectations from theory, I
did not find the predicted border effects in the population genetic structure of the
species examined. Gene flow estimates did not differ among central and peripheral
species. Genetic diversities were not lower in peripheral populations compared to
central populations or in species sampled towards the periphery compared to those
sampled in the centre of their ranges. Indeed, genetic diversities were much greater in
the peripheral species compared to their central counterparts. The distribution of genetic
variation indicated that secondary contact among differentiated lineages may, in part, be
responsible for the high genetic diversity in these peripheral species. Elevated mutation
rates mediated by environmental stress on the species’ margin may have contributed
further genetic variability in these species