Reproduction at the range limits of laminariales at the chilean and european coasts

Life history theory postulates that the schedule and duration of key events in an organism's lifetime are shaped by natural selection to produce the largest possible number of surviving offspring. These events, notable related to growth, reproduction and survivorship partly depend on the physical and ecological environment of the organism. The relative stability of the environment can lead to selection for certain life history trait (i.e. sexual vs asexual reproduction). Organisms can modify their reproductive strategies face to changing environments having impact in their species geographical distribution, especially at range edges.
Kelps (Phaeophyceae, Laminariales) are key structural components of benthic marine communities. Kelps display complex heteromorphic life histories that include haploid (spores and gametophytes) and diploid (embryonic sporophytes) microscopic life-stages and a diploid (adult sporophyte) macroscopic stage. While numerous studies have examined the ecology and physiology of these microscopic stages, our understanding of the geographic distribution of most kelp species comes primarily from studies on the adult macroscopic stages. Therefore, to fully understand how their range limits are regulated, studies in the diversity of reproduction and how that affects the micro- and early macroscopic life stages is imperative.
The global objective of this thesis is to compare the reproductive systems of Laminarian populations along both the centre of the species' range and in the limits of the species' distribution in order to study the ecological and evolutionary limits of the adaptation of species.
The first objective consisted in study sex ratio in two cryptic species of the Lessonia nigrescens complex. These species posses contrasting latitudinal distributions: one located north and the other south of the biogeographic boundary at latitude 29-30° S. Our results demonstrated that sex ratio in two cryptic species seems to be mainly genetically determined and environmental influence can significantly modify it. Second, a latitude effect was revealed but it was mainly explained by an increased variation in sex ratio at the range limits of species. This greater variation at the margins could be due either to differential mortality between sexes or to geographical parthenogenesis (asexual reproduction).
The second objective was to investigate the role of temperature in different developmental stages of the microscopic phases in the two cryptic species of Lessonia in order to understand the actual distribution of the Lessonia nigrescens species complex. Special attention was given to populations in the transition zone to compare to central populations of their respective species. Our results clearly demonstrate that there is differential tolerance to temperature in the two species, with the gametophytes of the Northern species being more tolerant to higher temperatures than gametophytes from the south. Second, the two species exhibit different life history strategies with a shorter haploid phase in the Northern species contrasted with considerable haploid vegetative growth in the Southern species. Third, the local temperature regime was a stronger factor than genotype (Southern or Northern species) in determining the temperature response of marginal populations.
The final objective was to study sexual reproduction in marginal populations of Laminaria digitata in the Brittany coast. Results showed that the environmentally unstable populations at the range limit of L.digitata displays a decline in genetic diversity compared to central populations. Surprisingly our results showed that sporophytes in these marginal populations gave mainly diploid spores rather than haploid ones suggesting apomeiosis or automixis events. Thes spores developed normally in culture. These results support the existence of geographical parthenogenesis in marine environments.