Diversity in the reproductive modes of European Daphnia pulicaria deviates from the geographical parthenogenesis

Abstract

Background: Multiple transitions to obligate parthenogenesis have occurred in the Daphnia pulex complex in North America. These newly formed asexual lineages are differentially distributed being found predominantly at high latitudes. This conforms to the rule of geographical parthenogenesis postulating prevalence of asexuals at high latitudes and altitudes. While the reproductive mode of high-latitude populations is relatively well studied, little is known about the reproduction mode in high altitudes. This study aimed to assess the reproductive mode of Daphnia pulicaria, a species of the D. pulex complex, from high altitude lakes in Europe.

Methodology/principal findings: Variation at eight microsatellite loci revealed that D. pulicaria from the High Tatra Mountains (HTM) had low genotype richness and showed excess of heterozygotes and significant deviations from Hardy-Weinberg expectations, and was thus congruent with reproduction by obligate parthenogenesis. By contrast, populations from the Pyrenees (Pyr) were generally in Hardy-Weinberg equilibrium and had higher genotypic richness, suggesting that they are cyclic parthenogens. Four lakes from lowland areas (LLaP) had populations with an uncertain or mixed breeding mode. All D. pulicaria had mtDNA ND5 haplotypes of the European D. pulicaria lineage. Pyr were distinct from LLaP and HTM at the ND5 gene. By contrast, HTM shared two haplotypes with LLaP and one with Pyr. Principal Coordinate Analysis of the microsatellite data revealed clear genetic differentiation into three groups. HTM isolates were intermediate to Pyr and LLaP, congruent with a hybrid origin.

Conclusion/significance: Inferred transitions to obligate parthenogenesis have occurred only in HTM, most likely as a result of hybridizations. In contrast to North American populations, these transitions do not appear to involve meiosis suppressor genes and have not been accompanied by polyploidy. The absence of obligate parthenogenesis in Pyr, an environment highly similar to the HTM, may be due to the lack of opportunities for hybridization.

Figure 1. Geographic location of sampling sites.

For the lowland populations (LLaP) each of the four sites is shown with a separate symbol while for the alpine populations single symbol is shown for each region (HTM and Pyr) where the sites were situated close to each other.

Figure 2. Principal Coordinate Analysis of microsatellite data.

The first two principal coordinate axes are shown that represent 24% and 14% of the variation. K-means groups are represented with polygons surrounding isolates from Pyr lakes (green triangles) and ponds (green squares), from HTM (red circles), and from LLaP (blue diamonds).

Figure 3. Maximum likelihood phylogeny of mtDNA haplotypes.

Statistical support for the major clades is expressed as the percentage bootstrap proportions of 1000 replicates and as the SH-like approximate likelihood ratio probabilities.

Figure 4. Phylogenetic relationships between obligate and cyclic parthenogens.

The dendrogram of European mtDNA haplotypes is as in Figure 3 (branch lengths not to scale), with branches color-coded to indicate where the haplotype was found in an obligate parthenogenetic or cyclic parthenogenetic population or in a population with mixed or uncertain reproductive mode (see Discussion).

Figure 5. Restriction map of the mtDNA ND5 amplicon.

The ApoI site at position 344 distinguishes species of the tenebrosa group (European Daphnia pulicaria, EuroPC; and D. tenebrosa, TENE) from those of the pulicaria group (polar, PPC; western, WPC; and eastern D. pulicaria, EPC; D. middendorffiana, MIDD; and panarctic D. pulex, PanPX). Some lineages yield short fragments under 100 bp in size difficult to visualize using agarose gels but this does not affect scoring the individuals as either tenebrosa or pulicaria group.