Daphnia pulicaria

Bellati, Adriana, Tiberti, Rocco, Cocca, Walter, Galimberti, Andrea, Casiraghi, Maurizio, Bogliani, Giuseppe & Galeotti, Paolo, 2014, A dark shell hiding great variability: a molecular insight into the evolution and conservation of melanic Daphnia populations in the Alps, Zoological Journal of the Linnean Society 171 (4), pp. 697-715 : 709-710

DAPHNIA PULICARIA IN THE ALPS

As already observed for other EuPC populations inhabiting remote freshwater ecosystems of western and eastern European mountains, the alpine haplotypes are extremely diversified and show genetic uniqueness compared with all of the other lineages. Indeed, both phylogenetic inference and haplotype network analyses suggest they are actually endemic to this mountain region, none of them occurring in any other European population considered ( Figs 2–4). The main outcome of our molecular analysis was however the discovery that our melanic populations belong to two deeply differentiated phylogeographical lineages (ALPS-1 and ALPS- 2, Fig. 2), which probably underwent alternative colonization patterns in this region after the start of the deglaciation of the Alps cirque lakes (fewer than 10 000 years ago). Indeed, despite the short geographical distances between our studied populations (5 km on average), the genetic differentiation amongst COI sequences found between melanic populations assigned to distinct lineages (2.4%) was at least twice as high as the estimated divergence between alpine and lowland haplotypes (1%).

Noteworthily, the analysis of the four melanic populations recently discovered within the GPNP, in the Western Italian Alps, allows us to draw wider conclusions concerning the previously hidden evolutionary history of rare alpine the D. pulex group populations. Indeed, melanic forms inhabiting the three alpine lakes located in the valley of the Dora di Savarenche river ( Fig. 1 ) show high genetic relatedness with circumpolar populations (all together constituting a clade that we tentatively named the ‘Boreal’ clade), whereas the only melanic populations located in the catchment of the river Orco (Lillet, Fig. 1 ) falls into a deeply differentiated ‘Alpine’ clade with more southern genetic affinities. This ancient alpine lineage probably diverged in a glacial refugium located in the same mountainous area, but at lower altitudes than the present distribution, as already suggested for the other ancient EuPC lineage inhabiting western European mountains (i.e. the PYR haplogroup). By contrast, alpine representatives of the ‘Boreal’ clade should derive from a northern ancestor, which spread southwards from the Arctic and then persisted only in relict populations in the Alps. So far, the closest extant populations of this boreal lineage (ALPS-1) are known from circumpolar islands ( Iceland and Svalbard), located over 2500 and 4000 km away from the European Alps, respectively ( Marková et al., 2013).

The occurrence of boreal haplotypes in southern European alpine lakes is remarkable as to date this has never been reported by previous authors. Nevertheless, the persistence of boreal elements resulting from ancient postglacial colonizations of European mountain lakes has been already reported for members of the D. longispina group, particularly Daphnia lacustris Sars, 1862 , from the High Tatra Mountains ( Petrusek et al., 2007) and the persistence of arctic-boreal elements is also well documented in both alpine flora and fauna (e.g. Mani, 1968). Alpine representatives of the ‘Boreal’ clade may derive from a rare case of a recent long-range dispersal of circumpolar EuPC individuals from their current distributional range, or (in a more fascinating scenario) from much earlier postglacial colonization patterns. Interestingly, the area surrounding the three lakes Nivolet and Trebecchi Superiore/ Inferiore is renowned for being regularly frequented by waders such as the Eurasian dotterel ( Charadrius morinellus Linnaeus, 1758 ) during the postbreeding migration towards their winter quarters in northern Africa ( Gruppo Piemontese Studi Ornitologici, 2013). As the breeding distribution of this arctic species reaches the northernmost part of Scandinavia (i.e. the Varanger peninsula, Norway; Lüker, Kraatz & Kraaz, 2011), we can speculate that at the end of glaciations migratory birds visiting alpine regions in southern Europe allowed the colonization of newly deglaciated lakes by arctic EuPC ancestors.

The substantial genetic variation observed within the ALPS haplogroup (i.e. also including populations considered in previous studies) can be explained by local postglacial diversification and suggests that lakes of the alpine region were colonized from genetically divergent source populations. The genetic distinctiveness of the lakes further suggests that ancient genotypes might have quickly monopolized local resources, generating a priority effect against subsequent colonists ( De Meester, 1996; De Meester et al., 2002). After ancestor populations were established, subsequent founder effects may have been major determinants of the present-day diversity, following a long-lasting expansion model of dispersal-by-colonization ( Orsini et al., 2013). According to this pattern of expansion, private haplotypes should occur in the majority of the ponds, although gene flow may also rarely occur at small spatial scales (e.g. amongst the three lakes in the Dora di Savaranche river catchment, Fig. 3 ). Although we have not measured the genetic variation within populations to assess the extent of gene flow amongst lakes, differences in morphological characteristics of various populations (i.e. occurrence of melanic phenotypes), coupled with the genetic distinction of mitochondrial haplotypes, suggest that founder effects and subsequent local adaptation have played an important role in shaping the present genetic structure of EuPC alpine populations ( De Meester et al., 2002; Ishida & Taylor, 2007). The proposed scenario of alternative colonization events could be further supported, at least for our melanic populations, by considering present zooplankton communities inhabiting distinct alpine lakes. Lake Lillet is indeed the only one in which pigmented individuals do not live in sympatry with the D. longispina group, but instead with a well-diversified zooplankton community ( Tiberti, 2011), whereas the others show very similar zooplankton communities ( Tiberti et al., 2013).

Figures

Figure 1. Map showing the distribution of European Daphnia pulicaria, including melanic populations specifically sampled in alpine lakes in the Western Italian Alps for this study. Within the enlarged box, the dashed grey line delimits river catchments where melanic populations were found (Orco and Dora di Savaranche). On the right, one melanic specimen from lake Nivolet. Abbreviations: GPNP, Gran Paradiso National Park. Abbreviations: SJM, Svalbard; RUS, Russia; ISL, Island; NOR, Norway; SWE, Sweden; GBR, Great Britain; DEU, Germany; POL, Poland; CZE, Czech Republic; CHE, Switzerland; ESP, Spain; HUN, Hungary; MNE, Montenegro; ALB, Albania; MKD, Macedonia; TRI, Trebecchi Inferiore; TRS, Trebecchi Superiore; NIV, Nivolet; LIL, Lillet.

Figure 3. Map of the Alps showing European Daphnia pulicaria populations (1–12) and haplotypes (A1–A10) considered in this study. Different colours and patterns identify haplotypes found in more than one population or together with other haplotypes. See Table 1 for details of populations.