Resting Egg Banks Research Articles

Patterns of genetic differentiation in resting egg banks of a rotifer species complex in Spain

Here we present the first population study on the genetic diversity of a rotifer species complex (Brachionus plicatilis) in a geographic area (Eastern Spain), which includes inland and coastal habitats. A novel approach was used by hatching resting eggs from sediment samples rather than sampling rotifers from the water column. Hatchlings from resting eggs sampled at 22 sites resulted in 434 clones which were screened using four polymorphic allozyme loci. A high level of genetic diversity was found consisting of 43 multilocus genotypes which were clustered into six well differentiated groups, often found in sympatry, with few or no hybrids between them. Mating experiments revealed preferences that supported the observed pattern of genetic differentiation. Some groups had restricted geographical distributions, and showed a preference for habitat type. Our results confirm patterns found in other zooplanktonic organisms and suggest that classical taxonomy has underestimated biological diversity in rotifers. The presence of additional cryptic sibling species in this complex seems very probable. Our study demonstrates the feasibility of using sediment sampling to explore genetic differentiation between species and populations, and shows the role of the resting egg bank in maintaining genetic diversity.

Phylogeography and regional endemism of a passively dispersing zooplankter: mitochondrial DNA variation in rotifer resting egg banks.

We investigated the phylogeography of the salt water rotifer Brachionus plicatilis, a cyclical parthenogen with passive dispersal mechanisms, using resting eggs recovered from saline lake sediments. Individual resting eggs were obtained from a large selection of lakes which were representative of five endorheic basins and the chain of coastal ponds in the Iberian Peninsula. The novel use of resting eggs allows the integration of seasonal and annual variations as well as the impact of stochastic effects such as drift and local extinction. A 653 bp fragment of the mitochondrial cytochrome oxidase subunit I (COI) gene was sequenced from 98 eggs. Our results revealed a deep phylogeographical structure in this species, with a division into two main lineages with distinct geographical distributions, which probably diverged at the beginning of the Pleistocene period. Most of the mitochondrial DNA haplotypes were restricted to single lakes. Nested clade analysis supported Early Pleistocene fragmentation of populations, low gene flow and some long-distance colonization. These conclusions contrast strongly with previous ideas on rotifer biogeography and this pattern is consistent with a recolonization of the Iberian Peninsula from two glacial refugia. The results provide new insights into the processes responsible for the genetic diversification of passive dispersers, a life-history trait typical of zooplanktonic biotas.

Molecular genetic analysis of Daphnia in the English Lake District: species identity, hybridisation and resting egg banks

Summary 1 Taxonomic issues in many Daphnia species complexes are often confused by a high degree of phenotypic plasticity and by interspecific hybridisation. Here, we employ molecular genetic tools to confirm the species composition and incidence of hybridisation in extant and resting egg populations of Daphnia from Windermere and Esthwaite Water in the English Lake District. 2 A combination of species-diagnostic allozymes and mtDNA, confirms that contemporary populations are dominated by a single species, D. galeata. 3 We present the first account of genetic characterisation of dated ‘resting’ eggs using microsatellites and mtDNA, employing PCR-based DNA recovery, thus providing a temporal dimension to taxonomic patterns. Thirty years ago, two species were present in Esthwaite, D. galeata and D. hyalina, but Windermere populations were dominated by D. galeata only. 4 The use of PCR-based mtDNA RFLP analysis as a species-diagnostic tool, and microsatellites to monitor clonal diversity, provide a valuable approach for long-term studies, especially in populations free from the complicating effect of frequent hybridisation. The detailed limnological records available for many large lakes, and associated changes in land-use, pollutants and climate, combined with long-term ephippial molecular genetic data, provide opportunities for exploring natural and anthropogenic impacts on genetic and community structure.

Sex, parthenogenesis and genetic structure of rotifers: microsatellite analysis of contemporary and resting egg bank populations.

Cyclically parthenogenetic rotifers are a valuable model for investigating the relationship between reproductive mode and population structure, although advances in this field have been hindered by low allozyme variability in these organisms. A high genotypic diversity is predicted after population establishment, which would be eroded by clonal selection during the parthenogenetic phase. The resting egg bank, produced sexually, is presumed to store high levels of genetic diversity, with subsequent effects on planktonic population structure. Here, we provide the first application of microsatellite markers to a rotifer planktonic population and its associated resting egg bank. Seven polymorphic microsatellite loci were screened in populations of the rotifer Brachionus plicatilis in a temporary pond to analyse: (i) the genetic structure of the resting egg bank; (ii) the changes in the genetic structure of rotifer populations during the parthenogenetic phase; and (iii) the population structure after its initiation from resting eggs. Microsatellites proved to be a useful tool for clone identification, revealing a surprisingly high clonal diversity in rotifer populations. The last sample in the parthenogenetic phase showed evidence of clonal selection, as indicated by a low observed clonal diversity and the appearance of linkage disequilibria. The resting egg bank, analysed comprehensively for the first time in any zooplankter, is in Hardy-Weinberg and linkage equilibrium, and contains a high genotypic diversity. Unexpectedly, the resting egg bank differed from the planktonic population in its allelic composition, suggesting that resting egg hatching is biased.