Abstract
The systematic biology of the subgenus Daphnia s . s . remains confused. Prior attempts at resolution used chiefly postabdominal claw morphology, chromosome numbers and rRNA gene sequences as characters for higher-level relations. Still, several taxa, such as Daphnia curvirostris Eylmann, 1878, have unclear affiliations. We addressed the position of D. curvirostris in this genus by estimating phylogenetic trees from a rapidly evolving protein coding gene (ND2), conducting broad geographical comparisons and carrying out detailed morphological comparisons. The Japanese ‘ curvirostris’ was found to be a new divergent lineage in the subgenus Daphnia , and to possess distinctive morphological characteristics from D. curvirostris . We described this new species as Daphnia tanakai sp. nov. , and redescribed D. curvirostris . The polymorphic postabdominal claw morphology and the distinctive chromosome number of D. tanakai sp. nov. provided evidence for rapid evolution of these traits. Our new morphological, chromosomal and genetic assessment of Daphnia weakened the argument for division of the subgenus Daphnia ( Daphnia ) O. F. Muller, 1785 sensu Johnson, 1952, into two further subgenera. © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 146 , 385‐405. ADDITIONAL KEYSWORDS: chromosome ‐ Daphnidae ‐ mitochondrial DNA ‐ morphology ‐ phylogeny ‐ postabdominal claw ‐ subgenera ‐ taxonomy.
Highlights
The taxonomy of the subgenus Daphnia remains notoriously confused
We addressed the phylogenetic position of D. curvirostris, and the agreement of the ND2 tree with the evolutionary groups predicted from claw morphology and chromosome number
Penton & Crease (2004) presented robust phylogenetic evidence that D. parvula is a member of pulex clade, whereas we have shown here that D. tanakai sp. nov. is a member of the D. longispina clade
Summary
The taxonomy of the subgenus Daphnia remains notoriously confused. Phenotypic plasticity (especially of head and carapace shape), hybridization, cryptic intercontinental introductions and poor taxonomic descriptions have hindered the understanding of species boundaries (Taylor & Hebert, 1993, 1994; Schwenk, Posada & Hebert, 2000). Most of divergent Daphnia species groups (as assessed by DNA sequence comparisons) have been recognized over 100 years ago (Brooks, 1957; Colbourne & Hebert, 1996). As detailed amongcontinent comparisons that assess both genetic and morphological variation in water fleas are still rare, more discoveries are certainly expected (Colbourne et al, 1998; Berny & Hebert, 1999). Many authors considered the postabdominal claw morphology or chromosome number to have undergone conserved evolution (Brooks, 1957; Beaton & Hebert, 1994; Colbourne, Hebert & Taylor, 1997), and formed the groups largely on this evidence. The proximal and medial pectens of the Daphnia pulex group have fewer, but more robust teeth than the pectens of the Daphnia longispina
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