Patterns and dynamics of rapid local adaptation and sex in varying habitat types in rotifers

Local adaptation is an important principle in a world of environmental change and might be critical for species persistence. We tested the hypothesis that replicated populations can attain rapid local adaptation under two varying laboratory environments. Clonal subpopulations of the cyclically parthenogenetic rotifer Brachionus calyciflorus were allowed to adapt to two varying harsh and a benign environment: a high-salt, a food-limited environment and untreated culture medium (no salt addition, high food). In contrast to most previous studies, we re-adjusted rotifer density to a fixed value (two individuals per ml) every 3–4 days of unrestricted population growth, instead of exchanging a fixed proportion of the culture medium. Thus our dilution regime specifically selected for high population growth during the entire experiment and it allowed us to continuously track changes in fitness (i.e., maximum population growth under the prevailing conditions) in each population. After 56 days (43 asexual and eight sexual generations) of selection, the populations in the harsh environments showed a significant increase in fitness over time relative to the beginning compared to the population in untreated culture medium. Furthermore, the high-salt population exhibited a significantly elevated ratio of sexual offspring from the start of the experiment, which suggested that this environment either triggered higher rates of sex or that the untreated medium and the food-limited environment suppressed sex. In a following assay of local adaptation we measured population fitness under “local” versus “foreign” conditions (populations adapted to this environment compared to those of the other environment) for both harsh habitats. We found significantly higher fitness values for the local populations (on average, a 38% higher fitness) compared to the foreign populations. Overall, local adaptation was formed rapidly and it seemed to be more pronounced in the high-salt treatment

Effect of diazinon on life stages and resting egg hatchability of rotifer Brachionus plicatilis

The effects of organophosphate pesticide, diazinon, on life history parameters and hatchability of resting eggs of rotifer Brachionus plicalitis were assessed. Newly hatched (<1h-old) neonates were individually cultured in six varying concentrations (0/control, 0.1, 1.0, 2.5, 5.0 and 10.0 mg/L) of diazinon. The life history parameters such as time (h) the rotifers bear first egg and release first neonate, reproductive period, net reproductive rate, mixis, intrinsic rate of population increase, and life span were evaluated. Results showed that among the life history parameters, the time the rotifers took to release neonates is the most sensitive, giving the lowest EC50 value of 1.24 mg/L. The fecundity of maternal females, amictic and mictic daughters was also investigated. Rotifers exposed to 10.0 mg/L produced significantly fewer amictic daughters, and at this concentration, rotifers did not produce any mictic daughter. At 5.0 mg/L, the number of male offspring was significantly lower than the control. Furthermore, the hatchability of resting eggs produced by the rotifers was evaluated when exposed to diazinon: from birth until they produced resting eggs (early development); during late developmental stage of resting eggs (before diapause); and during diapausing stage. The hatchability of the resting eggs was not affected when exposure was timed at late developmental and diapausing stages. Overall results showed that even though amictic females reproduced normally in the presence of low concentration of diazinon, sexual reproduction is severely affected, especially the hatchability of resting eggs when the exposure was timed on its early developmental stages. This provides another evidence that production of resting eggs is particularly sensitive to the presence of xenobiotics in the environment

Loss of Sexual Reproduction and Dwarfing in a Small Metazoan

Asexuality has major theoretical advantages over sexual reproduction, yet newly formed asexual lineages rarely endure. The success, or failure, of such lineages is affected by their mechanism of origin, because it determines their initial genetic makeup and variability. Most previously described mechanisms imply that asexual lineages are randomly frozen subsamples of a sexual population.We found that transitions to obligate parthenogenesis (OP) in the rotifer Brachionus calyciflorus, a small freshwater invertebrate which normally reproduces by cyclical parthenogenesis, were controlled by a simple Mendelian inheritance. Pedigree analysis suggested that obligate parthenogens were homozygous for a recessive allele, which caused inability to respond to the chemical signals that normally induce sexual reproduction in this species. Alternative mechanisms, such as ploidy changes, could be ruled out on the basis of flow cytometric measurements and genetic marker analysis. Interestingly, obligate parthenogens were also dwarfs (approximately 50% smaller than cyclical parthenogens), indicating pleiotropy or linkage with genes that strongly affect body size. We found no adverse effects of OP on survival or fecundity.This mechanism of inheritance implies that genes causing OP may evolve within sexual populations and remain undetected in the heterozygous state long before they get frequent enough to actually cause a transition to asexual reproduction. In this process, genetic variation at other loci might become linked to OP genes, leading to non-random associations between asexuality and other phenotypic traits

Rotifera 6: Asplanchnidae, Gastropodidae, Lindiidae, Microcodidae, Synchaetidae, Trochosphaeridae and Filinia

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