Morphometric and molecular (COX 1) variations of Asplanchna girodi clones from Central Mexico

Aim: In the past two decades, studies on cryptic speciation have increased rapidly leading to the need to reconsider cosmopolitanism in Rotifera. Cryptic species complexes are reported in some taxa of rotifers, especially Brachionidae and Epiphanidae. Less is known about Asplanchnidae although several morphotypes are known to exist. In this work, we report the morphometric and molecular aspects of Asplanchna girodi (Asplanchnidae). Methodology: The zooplankton samples were collected at 6 localities in central Mexico. We considered the individuals of A. girodi from different zones of a given waterbody as distinct populations and the same species from different geographical regions as distinct clones. For each A. girodi population. we obtained morphometric (body size and trophi measurements) and molecular (COX 1) data. Results: The clones from Xochimilco were larger in size (707 +/- 10 mu m) while the smallest sized individuals were from Azcapotzalco (640 +/- 16 mu m). Significant intrapopulational differences in the body size and trophi size were observed for half of the clones considered in this study (Toluca. Xochimilco and Zempoala). A total of 30 sequences were obtained for A. girodi; these were composed of 566 residues which 426 were conserved sites and 140 were variables. Comparisons among the interclonal populations of A. girodi showed that the clone from Toluca was the most divergent, up to 6.5%; the rest of the interclonal divergence values were <1.2%. Interpretation: The molecular marker COX 1 had superior resolution to distinguish to clones within A. girodi and to separate cryptic species. Molecular analysis also indicated the existence of a high genetic diversification process (K2P>6%) within the species. Thus it appears that A. girodi is a cryptic species complex.Further this study calls for a more integrative approach using both morphometric and molecular tools to decipher the existence of cryptic speciation in asplanchnids.