Speciation in ancient cryptic species complexes: evidence from the molecular phylogeny of Brachionus plicatilis (Rotifera)

Postembryonic development of hard jaws (trophi) in a species belonging to theBrachionus plicatilis complex (Rotifera, Monogononta): A morphometric analysis

The presence of hard jaws (trophi), with species-specific shape and size, is a taxonomic feature of Rotifera, a group of microscopic metazoans. Since trophi are used to discriminate among species, it is important to know whether these structures change in taxonomically important ways during postembryonic development. Using both SEM and optical images, we analyzed more than 100 individuals of a single clonal lineage of a monogonont rotifer, Brachionus plicatilis, in order to describe body and trophi development after hatching. Body size, expressed as lorica width and length, was isometrically related to age of the animals only during preadult stages. Trophi size, expressed as length of the different parts, was unrelated to either age or body size. Therefore, trophi elements do not grow after hatching in B. plicatilis. Despite the dimensional invariance with age, some differences in trophi size among individuals of the same clone were recorded. No difference in left-right asymmetry of the trophi was shown; the asymmetric elements of the trophi named rami consistently had the right ramus longer than the left. This constancy is in contrast to the reported trophi asymmetries in bdelloid rotifers, in which left-right asymmetries are not constant within clonal lineages. In conclusion, we suggest that also trophi size, constant within the analyzed clone, may be used as an additional taxonomic feature to help in the discrimination of taxa within the B. plicatilis complex of cryptic species.

Larval morphology, genetic divergence, and contrasting levels of host manipulation between forms of Pomphorhynchus laevis (Acanthocephala)

Studies on parasite species with a wide geographic and ecological range may be confounded by still equivocal taxonomic identification. Here, we investigated genetic polymorphism and behavioural changes induced in a common intermediate host, in two different forms of Pomphorhynchus laevis based on the morphology of the larval infective stage (cystacanth). A 'smooth type' (S) and a 'wrinkled type' (W) of cystacanth were distinguished based on their surface and shape. We analysed sequence divergence at both nuclear (ribosomal gene 18S rDNA, and ribosomal internal transcribed spacers, ITS1/ITS2) and mitochondrial (cytochrome c oxidase subunit 1) genes of P. laevis cystacanths and adults at various geographical scales. A high level of sequence divergence at ITS1, ITS2 and cytochrome c oxidase subunit 1 (11, 8 and 20%, respectively) was found between these two forms. The divergence pattern consistently discriminated two groups independently of geographic origin or host, and was congruent with larval morphology. The two forms also strongly differed in the intensity of behavioural change induced in their common intermediate host, Gammarus pulex, with the S-type parasite inducing a positive phototactism, whereas W-type infected gammarids were as photophylic as uninfected ones. Overall, our data strongly support the specific status of these two forms. We suggest that smooth cystacanths correspond to P. laevis, whereas wrinkled cystacanths might correspond to the previously described and poorly documented, Pomphorhynchus tereticollis, considered a synonym of P. laevis. This study also confirms the value of a joint analysis of internal transcribed spacers and cytochrome c oxidase subunit 1 genes to biogeographic studies on these species. Finally, we emphasize the importance of linking morphological and biological characteristics of acanthocephalan cystacanths to molecular data, in the study of the evolutionary ecology and systematics of this group.

Ecological and morphological differentiation among cryptic evolutionary lineages in freshwater limpets of the nominal form-group Ancylus fluviatilis (O.F. Müller, 1774)

The phylogeny and potential mode of speciation of the river limpet Ancylus fluviatilis (Basommatophora) was examined using mitochondrial DNA sequences from 16S ribosomal RNA, cytochrome c oxidase subunit I (COI) and nuclear DNA from internal transcribed spacer (ITS-1) regions from 103 populations across Europe. Four highly divergent lineages were observed within Ancylus. Clade 1, representing the nominal taxon Ancylus fluviatilis (O.F. Müller, 1774), is mainly found in central and northern Europe, Clade 2 is present in a single Portuguese population, Clade 3 is distributed on the Canary islands, North Africa and the eastern Mediterranean region, whereas Clade 4 inhabits the Northern Mediterranean coasts. Phylogenetic analyses revealed an overall consistent topology of nuclear and mitochondrial gene trees. Based on a molecular clock, we estimated that the basic radiation occurred in the late Pliocene. Although clades differ significantly in size independent shell shape, morphological differentiation of lineages is not feasible without genetic data. Environmental data related to climate (precipitation, temperature, etc.) showed a significant differentiation of clades. Clade 1 dwells in relatively colder and more stable habitats than Clades 3 and 4, whose habitats in turn differ in a low or high amount of precipitation during spring and autumn, respectively. Based on the combined data sets on mitochondrial DNA, nuclear DNA, morphological and ecological differentiation, we conclude that Ancylus represents a cryptic species complex of reproductively and genetically isolated lineages. In addition, the joint analysis suggests that ecological speciation is probable to explain current patterns.

The recent evolutionary origin of the phenotypically novel amphipod Hyalella montezuma offers an ecological explanation for morphological stasis in a closely allied species complex

Numerous molecular studies have identified morphologically cryptic, freshwater invertebrate species, but have not suggested possible mechanisms for their phenotypic stasis. The amphipod crustacean genus Hyalella contains numerous morphologically cryptic species in the H. azteca complex, as well as a small number of morphologically very divergent, narrowly endemic taxa. One such taxon, Hyalella montezuma, is the sole planktonic filter-feeder within the North American amphipod fauna, and is known only from Montezuma Well, a fishless travertine spring mound in Arizona, USA. In this study, we conduct a phylogenetic analysis of mtDNA sequence data using likelihood, Bayesian and cladistic approaches to determine both the relationship of H. montezuma to the H. 'azteca' species complex, and to ascertain if its morphological and ecological differentiation have been comparatively recent. The results show that H. montezuma has a very close phylogenetic affiliation with one lineage in the H. azteca complex, indicating that its origin has been recent. We present evidence suggesting that fish predation is an important ecological factor, which constrains morphological and ecological diversification within the genus Hyalella, and that Montezuma Well has provided a relaxation on this constraint.