Contribution of cyclic parthenogenesis and colonization history to population structure in Daphnia

Abstract

Cyclic parthenogenesis, the alternation of parthenogenetic and sexual reproduction, can lead to a wide scope of population structures, ranging from almost monoclonal to genetically highly diverse populations. In addition, sexual reproduction in aquatic cyclic parthenogens is associated with the production of dormant stages, which both enhance potential gene flow among populations as well as impact local evolutionary rates through the formation of dormant egg banks. Members of the cladoceran genus Daphnia are widely distributed key organisms in freshwater habitats, which mostly exhibit this reproduction mode. We assessed patterns of genetic variation within and among populations in the eurytopic and morphologically variable species Daphnia longispina, using data from both nuclear (13 microsatellite loci) and mitochondrial (partial sequencing of the 12S rRNA gene) markers from a set of populations sampled across Europe. Most populations were characterized by very high clonal diversity, reflecting an important impact of sexual reproduction and low levels of clonal selection. Among-population genetic differentiation was very high for both nuclear and mitochondrial markers, and no strong pattern of isolation by distance was observed. We also did not observe any substantial genetic differentiation among traditionally recognized morphotypes of D. longispina. Our findings of high levels of within-population genetic variation combined with high among-population genetic differentiation are in line with predictions of the monopolization hypothesis, which suggests that in species with rapid population growth and potential for local adaptation, strong priority effects due to monopolization of resources lead to reduced levels of gene flow.

Identifying hybridizing taxa within the Daphnia longispina species complex: a comparison of genetic methods and phenotypic approaches

“…Hybridizing species of the complex can be well distinguished when several microsatellite loci are analyzed Thielsch et al, 2009), confirming the suitability of these markers for the identification of hybrid genotypes. Additionally, DNA microsatellite analysis provides detailed data on clonal composition and genotypic richness and is therefore particularly suitable for evolutionary studies .…”

“…However, recent studies (Petrusek et al, 2008a;Thielsch et al, 2009) suggest that the lakeinhabiting form D. hyalina, the mostly pond-inhabiting D. rosea, as well as various intermediate morphotypes just represent morphs of the single phenotypically variable biological species D. longispina. This conclusion is supported by the lack of genetic divergence in various molecular markers, such as mitochondrial DNA (Petrusek et al, 2008a), allozymes (Gießler et al, 1999), 13 unlinked microsatellite loci (Thielsch et al, 2009), and nuclear ITS sequences (Gießler & Englbrecht, 2009). Individuals identified as D. longispina from localities sampled in this study included typical D. hyalina-like morphotypes as well as transitional forms closer to D. rosea morphology.…”

“…Amplification and length assessment of the fragments mostly followed the protocol by Brede et al (2006) and Thielsch et al (2009). After amplicons had been obtained, they were appropriately diluted and electrophoresed on a CEQ 2000 capillary sequencer (Beckman Coulter, Fullerton, CA, USA) with self-designed size standards based on Lambda virus DNA (Symonds & Lloyd, 2004).…”

“…Gene flow among parental species may cause reticulate patterns of evolution which potentially lead to increased levels of genetic and morphological variability in local populations (Schwenk & Spaak, 1997). Although some studies suggest reticulate evolution plays an important role for the D. longispina complex Gießler et al, 1999;Gießler & Englbrecht, 2009), gene pools of parental species seem to remain distinct , and might be well distinguished by various molecular methods (e.g., Thielsch et al, 2009).…”

First record of planktonic crustaceans in Sardinian reservoirs

“…We observed species of the D. pulex group (putative D. pulex Leydig, 1860; after Margaritora 1983, see below) and of the D. longispina group (as defined in Petrusek et al 2008b;Thielsch et al 2009) with parental species and hybrids. Previously, D. pulex had only been reported from fishless temporary ponds in Sardinia; this is the first record of the presence of this species in lakes with fish like the Sardinian reservoirs.…”