The evolution of maturation in Daphnia

Maturation is a key life history transition for many organisms, due to the importance of age and size at maturity in determining fitness. Understanding how maturation phenotypes evolve requires an appreciation of the underlying ontogenetic mechanisms, including the maturation threshold, which determines when an individual ‘decides’ to mature. Maturation thresholds are poorly understood, and little is known about how phenotypically plastic or genetically variable they are, or how variable thresholds influence fitness. In this thesis the evolution of maturation thresholds is investigated using the crustaceans Daphnia magna and D. pulex. A comprehensive approach to modelling the maturation process found that the maturation threshold was a developmentally plastic trait in response to variable resource availability, and more closely resembled a process with a rate than a discrete switch. The maturation threshold also differed between genotypes for both species, and these differences were more apparent in D. magna than D. pulex. A second study of maturation in D. magna identified clone-specific parental effects in the threshold. Furthermore, these parental effects influenced growth, and reaction norms for age and size at maturity were a product of interacting effects between both growth and maturation threshold. A microarray study of gene expression changes in D. pulex found that most gene expression changes during maturation were continuous, further supporting the idea that the threshold is better thought of as a rate than a switch. This study also identified increases in vitellogenin transcripts, indicating the allocation of resources towards reproduction, and potential mechanisms for epigenetic inheritance and endocrine control of maturation. Finally the fitness consequences of variation in the maturation threshold were investigated in D. magna. Genotypes with a smaller threshold had a higher intrinsic rate of population increase, but threshold size did not correlate well with competitive success when five clones were directly competed with each other, suggesting that interactions with other factors were influencing fitness. The findings of this thesis suggest that maturation thresholds are not based on a single fixed state, but are responsive to environmental variation. The presence of heritable variation and transgenerational effects in these developmentally plastic traits suggests an important and adaptive role for them in the evolution of age and size at maturity

New insights on the population genetic structure of the great scallop (Pecten maximus) in the English Channel coupling microsatellite data and demogenetic simulations.

International audienceThe great scallop (Pecten maximus) is a commercially important bivalve in Europe, particularly in the English Channel, where fisheries are managed at regional and local scales through the regulation of fishing effort. In the long term, knowledge about larval dispersal and gene flow between populations is essential to ensure proper stock management. Yet, previous population genetic studies have reported contradictory results. In this study, scallop samples collected across the main fishing grounds along the French and English coasts of the English Channel (20 samples with temporal replicates for three sites,n= 1059 individuals), and the population genetic structure was analysed using 13 microsatellite loci. Coupling empirical genetic data with demogenetic modelling based on a biophysical model simulating larval exchanges among scallop beds revealed a subtle genetic differentiation between south-west English populations and the rest of the English Channel, which was consistent with larval dispersal simulations. The present study provides a step forward in the understanding of great scallop population biology in the English Channel, underlining the fact that even in a context of potentially high gene flow and recent divergence times since the end of the last glacial maximum, weak but significant spatial genetic structure can be identified at a regional scale

Offspring Provisioning Explains Clone-Specific Maternal Age Effects on Life History and Life Span in the Water Flea, Daphnia pulex

Genetic inheritance underpins evolutionary theories of aging, but the role that non-genetic inheritance plays is unclear. Parental age reduces the lifespan of offspring in a diverse array of taxa but has not been explained from an evolutionary perspective. We quantified the effect that maternal age had on the growth and maturation decisions, life history, rates of senescence and lifespan of offspring from three Daphnia pulex clones collected from different populations. We then used that data to test general hypotheses proposed to explain maternal age effects on offspring lifespan. Three generations of breeding from young or old mothers produced dramatic differences in the life-histories of fourth generation offspring, including significant reductions in lifespan. The magnitude of the effect differed between clones suggesting that genetic and non-genetic factors ultimately underpin trait inheritance and shape patterns of aging. Older parents did not transmit a senescent state to their offspring. Instead, offspring from older ancestors had increased early-life reproductive effort, which resulted in an earlier onset of reproductive senescence, and an increased rate of actuarial senescence that shortened their lifespan. Our results provide a clear example of the need to consider multiple inheritance mechanisms when studying trait evolution

Dmagna_ME_growth_PMRN

Data file for calculating and plotting probabilistic maturation reaction norms (PMRNs), for use with R scripts 5 and 6

DaphniaME_6of6_PMRN-figs

R script 6 of 6: Using best fitting GLM (identified in R script 5) in plots of probabilistic maturation reaction norms

DaphniaME_2of6_Univariate-figs

R script 2 of 6: Plotting univariate figures (those considered in the analysis of variance /GLMs from R script 1)

DaphniaME_1of6_MANOVA-ANOVA-posthoc

R script 1 of 6: calculating growth coefficients, followed by multivariate analysis of variance (MANOVA), analysis of variance (ANOVA)/GLMs and post-hoc tests

Dmagna_ME_alltraits_summary

Summary data file containing trait information for all individuals, for use with R scripts 1, 2, 3 and 4

DaphniaME_5of6_PMRN_GLM-selection

R script 5 of 6: Using GLMs to statistically model probabilistic maturation reaction norms, and comparing among models to identify the best fit

DaphniaME_4of6_PCV-stats

R script 4 of 6: Statistical analysis for identifying significant differences in vector lengths and angles of phenotypic change vectors (as visualised in R script 3)