Daphniahybridization along ecological gradients in pelagic environments: the potential for the presence of hybrid zones in plankton

Abstract

The relative homogeneity of pelagic environments has been regarded as the reason for the absence of hybrid zones for hybridizing planktonic Daphnia (Crustacea: Cladocera); occasional dominance of interspecific hybrids over parental species was explained by their temporal superiority in fluctuating environments. However, water bodies with spatially varying environmental conditions might facilitate the formation of hybrid zones in plankton. We studied the distribution of species and hybrids of the Daphnia longispina complex in 11 canyon-shaped reservoirs, localities characterized by horizontal environmental gradients (particularly of food supply and size-selective predation); we also analysed patterns of carapace size and fecundity among coexisting taxa. Spatial distribution of taxa agreed with their ecological characteristics; those showing different affinities along longitudinal reservoir profiles differed in size according to the presumed fish predation gradient. Only hybrids of Daphnia galeata with Daphnia cucullata and D. longispina (Zhyalina) were recorded. The latter two species preferred opposite ends of gradients, such spatial segregation probably explaining the absence of their hybrids. Distributional patterns were relatively stable in two consecutive summers, apart from a substantial decline of D. galeata!cucullata in the second year. The observed pattern of a hybriddominated zone in intermediate conditions suggests that local Daphnia hybrid zones may indeed form within reservoirs.

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

“…Allozymes, microsatellites, and ITS-RFLP analyses split the data set into five taxa: D. galeata, D. longispina, D. cucullata, D. galeata 9 longispina hybrids, and D. galeata 9 cucullata hybrids. No individual representing a D. longispina 9 cucullata hybrid was present in the data set (see also Petrusek et al, 2008b). In general, the molecular markers corresponded well to each other ( Table 2).…”

“…In studies on hybridizing Daphnia, it is even more crucial, as some mismatch of the markers can be expected (Schwenk & Spaak, 1995). In our previous studies of the D. longispina complex Petrusek et al, 2008b), we showed that unexpected intraspecific variation may substantially influence the interpretation of restriction patterns from ITS-RFLP (Billiones et al, 2004). As no detailed study has directly compared results of various molecular approaches for taxon identification in this species complex so far, the real extent of these discrepancies remains unknown.…”

“…Amplicons were digested by overnight incubation with the restriction enzymes MbiI and Eco52I (Fermentas, Burlington, Canada) and electrophoresed on an agarose gel. The banding patterns were interpreted according to Skage et al (2007) and Petrusek et al (2008b). Individuals exhibiting clear additive patterns were scored as hybrids even if the bands varied in intensity.…”

“…The canyonlike morphology of studied reservoirs results in longitudinal environmental gradients offering diverse microhabitats for zooplankton within individual water bodies. Such conditions promote the co-occurrence of parental Daphnia as well as hybrids Petrusek et al, 2008b) and therefore make the reservoirs excellent localities for studies on these hybridizing taxa.…”

“…Despite a small number of fixed diagnostic loci and sampling limitations imposed by the need to deep-freeze samples, this method has become widespread in studies on Daphnia hybridization (e.g., Spaak, 1996;Spaak, 1997;Gießler, 2001;Winder et al, 2001;Keller & Spaak, 2004). It still remains in use and continues to provide valuable results Keller et al, 2008;Petrusek et al, 2008b). However, alternative methods for taxon identification have been recently developed, for example ITS-RFLP, the restriction fragment length polymorphism (RFLP) of the internal transcribed spacer region (ITS) of nuclear ribosomal DNA (Billiones et al, 2004;Skage et al, 2007).…”