Abstract
Recent findings suggest a convergence of time scales between ecological and evolutionary processes which is usually explained in terms of rapid micro evolution resulting in evolution on ecological time scales. A similar convergence, however, can also emerge when slow ecological processes take place on evolutionary time scales. A good example of such a slow ecological process is the colonization of remote aquatic habitats by passively dispersed zooplankton. Using variation at the protein coding mitochondrial COI gene, we investigated the balance between mutation and migration as drivers of genetic diversity in two Branchipodopsis fairy shrimp species (Crustacea, Anostraca) endemic to remote temporary rock pool clusters at the summit of isolated mountaintops in central South Africa. We showed that both species colonized the region almost simultaneously c. 0.8 My ago, but exhibit contrasting patterns of regional genetic diversity and demographic history. The haplotype network of the common B. cf. wolfi showed clear evidence of 11 long distance dispersal events (up to 140 km) with five haplotypes that are shared among distant inselbergs, as well as some more spatially isolated derivates. Similar patterns were not observed for B. drakensbergensis presumably since this rarer species experienced a genetic bottleneck. We conclude that the observed genetic patterns reflect rare historic colonization events rather than frequent ongoing gene flow. Moreover, the high regional haplotype diversity combined with a high degree of haplotype endemicity indicates that evolutionary- (mutation) and ecological (migration) processes in this system operate on similar time scales.
Highlights
Inselbergs are isolated rocky outcrops rising abruptly from the landscape
Besides harboring a peculiar terrestrial fauna and flora [5], the formation of eroded depressions near the summit of these mountains often gives rise to unique aquatic ecosystems that intermittently hold water after rains [6]. These archipelagos of temporary rock pools situated on isolated mountaintops form a double insular habitat structured at three spatial scales with pools grouped in clusters, one or more pool clusters present on each inselberg and different inselbergs embedded in a matrix of inhospitable dryland [7]
Spatial structuring of genetic variation Current results based on mitochondrial sequence data suggest that, for rock pool inhabiting fairy shrimp, most genetic variation seems to be partitioned at the regional scale between inselbergs and to a lesser extent between different pool clusters
Summary
Inselbergs are isolated rocky outcrops rising abruptly from the landscape. Often occuring as monoliths, they are characterized by contrasting and often more extreme environmental conditions compared to the surrounding plains [1], [2]. Candidate vectors mediating long distance dispersal of fairy shrimp dormant eggs between distant pool clusters and inselbergs include birds [16] and wind [14], [17]. Genetic studies in different fairy shrimp species, for example, support the idea of frequent dispersal and gene flow over short distances such as within pool clusters [13], [22], [23], [24], [25]. Surprisingly little is known about the frequency of dispersal and gene flow among remote or isolated aquatic habitats such as mountain pools or groundwater aquifers and over large spatial scales [26], [27], [28]. A suitable approach to detect historic long distance dispersal (LDD) events is to study the geographic distribution of genetic lineages [29], [30]
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