Doctor of Philosophy
Department of Biological Sciences
Hunt, Alison, Effects of contrasting dispersal strategies on populations of two species of intertidal starfish: patiriella calcar and patiriella exigua, Doctor of Philosophy thesis, Department of Biological Sciences, University of Wollongong, 1993. http://ro.uow.edu.au/theses/1045
Despite the importance of dispersal in regulating patterns of gene flow, there are still major gaps in our knowledge of the consequences of many dispersal strategies for the genetic structure of marine populations. Until recently, most of our knowledge was based on studies which had investigated sexually reproducing species with the potential for wide dispersal of planktonic larvae. Relatively few studies have concentrated on intertidal species which lack a clear means of dispersal.
This study used ecological and genetic techniques to investigate the consequences of restricted dispersal for marine intertidal populations, and to provide an important comparison for two closely related co-occurring intertidal species with contrasting life histories. Importantly, this study also provides the first investigation of the genetic consequences of wide dispersal for an intertidal species over such a large area of the eastern Australian coastline. Additionally this is one of the first investigations of the consequences of restricted dispersal on natural populations along the east coast of Australia.
The co-occurring intertidal starfish, Patiriella calcar and P. exigua occur along much of the southeastern coastline of Australia. Whilst the breeding system is far from fully described for either species, P. calcar is believed to have the potential for wide dispersal via a sexually produced planktonic larvae. In contrast, P. exigua lacks a dispersive phase, as juveniles develop directly on the rocky shore and are thought to remain within the parental habitat. P. exigua's breeding system is further complicated as it is a simultaneous and/or protandric hermaphrodite.
Although the life history of P. calcar includes planktonic larvae, the broad geographic range over which sampling was conducted in this study (1,800km) provided the potential for genetic differentiation, as a consequence of linear dispersal along the coastline, and the complexity and unpredictability of the East Australian Current.
Despite the potential for disjunct genetic patterns among local populations of P. calcar, there was remarkably little genetic differentiation among local populations of P. calcar (FST = 0.008 ± 0.001). Such homogeneity was indicative of strong gene flow over the 1,800km of this study, and it was estimated that approximately 31 genetically effective migrants are exchanged between populations each generation. Electrophoretic data also indicated that local populations of P. calcar are maintained by recruitment of outcrossed sexually produced offspring. Single-locus genotype frequencies at the five enzyme-encoding loci assayed in 26 local populations, closely matched expectations for Hardy-Weinberg equilibria.
Like P. calcar, local populations of P. eXlgua are maintained by outcrossed sexual reproduction. Single-locus genotype frequencies of six enzyme-encoding loci within all 25 local populations of P. exigua were in close agreement with expectations for Hardy-Weinberg equilibria.
In contrast, to the broad panmixis of P. calcar and as expected for a species lacking a dispersive phase, large genetic differences were found among populations of P. exigua some separated by as little as 5km. Such genetic differentiation among populations resulted in an extremely high FST value of 0.518 ± 0.054 among 25 local populations over 1,1 00kn1 of the southeast Australian coastline. A significant positive correlation existed between geographic and genetic distance and was indicative of isolation by distance such as that predicted by a stepping stone model, implying that local populations may be at, or near, equilibrium under their current regime of dispersal. For P. exigua, the absence of gametic phase disequilibrium and the uniformity of genetic diversity among local populations further suggests that these may be well established local populations which have not recently been subject to population bottlenecks or recent colonisation.
Effectively closed populations such as those of P. exigua should be particularly susceptible to the effects of stochastic processes, variability in demographic parameters and changes in localised site-specific selection. However, the consistent pattern of polymorphism for alleles at the Mdh locus in a group of four local populations along the Victorian coastline, that are virtually fixed for alternate alleles in populations to the north and south, suggests that sporadic migration of individuals via rafting may have occurred among the northern and southern groups. Clearly, such stability results from the lack of genetic input from outside sources and the apparently large size of P. exigua local populations. However, localised site-specific selection for heterozygosity was put forward as a possible hypothesis to explain the unusual patterns of allelic variation at the normally monomorphic Mdh locus in four neighbouring populations on the Victorian coast. A set of breeding experiments were used to confirm the assumption of normal Mendelian inheritance at the Mdh and Gpi loci, as such an assumption is central to population genetic studies, especially in view of the unusual allelic patterns of variation at the Mdh locus.
In contrast to the extreme genetic differentiation exhibited among populations of P. exiuga is evidence that local populations are randomly mating, panmictic units (sensu Wright 1943) (Fsp values ranged from 0 to 0.020 within four local populations). Although individuals of P. exigua were not evenly distributed across the rocky shore, these aggregations were not family breeding groups, as all individuals were inferred to be the product of outcrossed sexual reproduction. The scale and extent of differentiation within populations will reflect the breeding system of the organism, the distance and frequency of migration within local populations and the frequency of recruitment from outside sources. These data imply that these four populations are apparently large, well established local populations which rarely receive successful immigration from outside sources, and that adults and juveniles freely mix and mate at random. A consequence of the seemingly large size of local populations, is that genetic drift is unlikely to be important unless Ne fluctuates significantly.
The contrasting life histories of P. calcar and P. exigua, resulted in vastly different patterns of genetic differentiation. Despite the potential barriers to dispersal of P. calcar, high levels of gene flow occurred among long populations along the east coast of Australia. As expected for a species with severely restricted dispersal, there was little gene flow among P. exigua populations. However, unlike the typical situation expected for species with severely restricted dispersal, local populations of P. exigua are large panmictic breeding groups in which apparently highly mobile adults and juveniles provide for gene flow. Consequently, these populations may be somewhat buffered against the effects of drift through fluctuations in population size.