Valeria Rossi

Persistence of asexuality through mixed reproduction in Eucypris virens (Crustacea, Ostracoda).

The ostracod species Eucypris virens exhibits geographical parthenogenesis, with rare sexual populations in southern Europe and widespread asexual populations elsewhere. DNA sequence data from the nuclear ITS1 and mitochondrial COI regions have been used to estimate genetic variabilities and reconstruct phylogenies. The observed divergence was exceptionally high, with intraspecific maxima of 10.3% (ITS1) and 20.9% (COI) among European lineages, levels reported for interspecific comparisons of other taxa. Phylogenetic reconstructions reveal multiple origins of asexual clones from sexual populations. However, we argue that such data can only provide a lower limit on the number of origins of asexual reproduction, and an upper limit on the age of asexual lineages. Congruence between gene trees for different loci can provide support for the inference of long-term apomictic reproduction. Nuclear and mitochondrial data differ in their placement of some asexual clones, possibly indicating that genetic exchange has taken place between sexual and asexual lineages. Such intraspecific hybridization is one route to combine the benefits of both reproductive modes, and it might explain how asexuality managed to persist in E. virens even in long, evolutionary terms.

Intraindividual and intraspecies variability of ITS1 sequences in the ancient asexual Darwinula stevensoni (Crustacea: Ostracoda)

The lack of variability in ITS regions within individuals and within species has been explained as the result of concerted evolution. In fact, many examples of intraindividual variation in the ITS regions have been reported. Here we report evidence of within-individual variation of the ITS1 region in the obligate parthenogenetic species Darwinula stevensoni. We analysed 46 clones obtained from 12 individuals of D. stevensoni, from three Italian sites and one site in Luxembourg. Seven nucleotides out of 366 were variable. Most variability (80%) was found among clones within individuals, and the remainder of the variability was observed among individuals. No difference was found among populations or between habitats. The low intraspecific variability and the observation of recombinant molecules are evaluated in light of the relevant literature. The high percentage of variation within individuals and the occurrence of recombination without meiosis are discussed by considering the ancient asexual ‘status’ of the species.

The clonal ecology of Heterocypris incongruens (Ostracoda)

A seasonal cycle in the frequency of electrophoretically distinguishable clones has been followed from late July 1985 to early August 1987 in an obligate parthenogenetic population of Heterocypris incongruens (Ramdohr) in a rice field located in the Po plain (Italy). Different clones predominate at different seasons. Summer and winter allozyme clones show size differences: otherwise the clones show no distinctive morphological features. The dimensional nature of the observed differences has been confirmed by multivariate analysis of 19 quantitative morphological traits. The electrophoretic clones reared in the laboratory at different temperatures (4?, 16?, 24?, 28?, 32?C) have survival curves and relative abundances that vary with temperature. In the laboratory, the clone dominant in the field in winter fares better at low temperatures while the clone dominant in the summer does so at high temperatures. Temperature is a likely selective factor responsible for the observed seasonal cycles in the field.

Phylogenetic analysis of Macrobiotidae (Eutardigrada, Parachela): a combined morphological and molecular approach

Combined analyses of morphological and molecular data were used to resolve phylogenetic relationships within Macrobiotidae (Eutardigrada). Morphological data were analysed using a cladistic approach with a matrix comprising 15 taxa with 17 characters to obtain a phylogenetic reconstruction. Molecular data were obtained by sequencing the cytochrome c oxidase subunit I gene in seven species of Macrobiotidae and one of Eohypsibiidae (used as outgroup). The morphological character defining the family, symmetrical claw on each leg, turns out to be plesiomorphic. Moreover, neither morphological nor molecular analyses supports a monophyletic clade for the subfamily Macrobiotinae, whereas both support a well defined evolutionary line (Murrayinae) within the family. We propose elevating the latter to family level (Murrayidae) while temporarily retaining as valid the family Macrobiotidae (amending its diagnosis, including within it only Macrobiotinae). Murrayidae opens an interesting evolutionary prospective, because the entire line has differentiated without sexual reproduction, constituting an example of evolution of asexual lineages.

Ontogenetic changes in the carapace shape of the non-marine ostracod Eucypris virens (Jurine)

Developmental changes in carapace form (size+shape) during ontogeny have been explored in Eucypris virens (Crustacea, Ostracoda) using elliptic Fourier analysis. Clones from different geographic localities raised under controlled constant conditions (temperature and photoperiod) were used to characterize developmental pathways in the species. A larger data set including field populations and laboratory populations cultured under a range of environmental conditions were used to infer influence of environmental factors on carapace shape changes during ontogeny. Size changes between consecutive juvenile stages support empirical laws describing the doubling of ostracod volume at each moult. Ontogenetic changes point out the remarkable influence of environmental conditions on carapace shape.

Egg diapause and clonal structure in parthenogenetic populations of Heterocypris incongruens (Ostracoda)

A study of the clonal structure of parthenogenetic populations of Heterocypris incongruens from rice-fields in Northern Italy carried out over two-year period is summarized. Significantly different levels of genetic polymorphism were found among populations. The coexistence of different electrophoretic clones and similar patterns of clonal seasonal succession have been observed in at least two different rice fields. The clone or group of clones present in fall, winter and early spring is substituted by other multilocus genotypes in late spring and summer. Different egg diapause induction mechanisms drive the clonal substitution in different clones. Here we report a new laboratory experiment designed to test the effect of temperature and photoperiod found in winter (12 °C 8:16 L:D), spring (24°C 12:12 L:D) and summer (28°C 16:8 L:D) conditions on the deposition and hatching of diapausal eggs in different multilocus genotypes. Clones respond in a way compatible with their sustained presence in the field in different seasons.

Heterocypris (Crustacea: Ostracoda) from the Isole Pelagie (Sicily, Italy): Hatching phenology of resting eggs

Abstract The hatching phenology of Heterocypris resting eggs was investigated in laboratory controlled conditions. Microcosms were set up using dry sediments from three freshwater temporary ponds (Cavallo Bianco, Aria Rossa and Vallone della Forbice) of the island of Lampedusa (Sicily) and kept at different temperatures and photoperiods. Hatching time was modulated by different environmental factors. Initial anoxia, induced by the decomposition of the sediments organic matter, delayed hatching in the microcosm set up with sediments from Vallone della Forbice which has a high organic matter content. Temperature and photoperiod in isolation did not seem to affect hatching significantly, while their interaction did. The coexistence of two female morphotypes with different reproductive modes could not be explained by a differential response to temperature and photoperiod. Coexistence was probably due to disturbance events, such as drying up of ephemeral pools. Some resting eggs (in different percentages in different microcosms) hatched only after a second hydration (after microcosms dried up). Hatching phenology is discussed in relation to genetic differences in egg responsiveness to hatching stimuli and to a bet‐hedging strategy in a year‐to‐year highly variable environment such as the ephemeral ponds of Lampedusa.

Heterocypris (Crustacea, Ostracoda) from the Isole Pelagie (Sicily, Italy): the coexistence of different morphotypes

Abstract Differences exist in the population structure of Heterocypris in five temporary ponds on the islands of Lampedusa and Linosa (Isole Pelagie, south of Sicily). In a temporary pond on Lampedusa (Cavallo Bianco) we found only H. incongruens females. All the other populations were bisexual, with the female/male sex ratio varying from 1 to 18. We analysed and measured a sub‐sample from each population and observed, as a typical morphological feature of adult females in the bisexual populations, a lobe‐like expansion of the selvage (lamella hyalina) on the posterior margin of the left valve. In another temporary pond from Lampedusa (Vallone della Forbice) both females with this lamella and others without it occur. We discuss the possible coexistence of two lineages of H. incongruens each having a different reproductive mode. The parthenogenetic lineage comprises females without a lamella hyalina whereas the bisexual females possess this feature. Alternatively, we hypothesise the sympatry of two different species of Heterocypris: H. incongruens and H. barbara. In this hypothesis, H. incongruens is parthenogenetic and H. barbara is amphimictic. Differences in mean body size between the adult females with and without the lamella hyalina are in accordance with values reported in the literature for niche differentiation among sympatric taxa.

Differences in demographic parameters among electrophoretic clones of Daphnia obtusa Kurz (Crustacea: Cladocera)

In an 18-month electrophoretic study, a Daphnia obtusa population that recently colonized Lake Orta (a large, severely polluted lake in Northern Italy) showed no genetic variation.

A preliminary study in the use of RAPD markers in detecting genetic differences in hatching patterns of Chirocephalus diaphanus Prévost, 1803 (Crustacea: Anostraca)

Randomly amplified polymorphic DNA (RAPD) was used to detect intra-clutch genetic differences of the anostracan Chirocephalus diaphanus Prévost, 1803. Thirty primers were tested on 130 nauplii from three different clutches. Two primers produced five repeatable and well-defined amplification products, that were used as genotype markers. Nauplii hatched after three successive dehydration and rehydration cycles. Consistent genetic differentiation was observed among nauplii from the same clutch. Significant association between presence or absence of three polymorphic amplification products and hatching after different stimuli was observed. These results suggest that RAPD markers might be related to the difference in nauplii hatching and provide reasonable support to the hypothesis that the source of the observed variation in hatching phenology are genetic as well as epigenetic.