Abstract
A major determinant of the geographic distribution of a species is expected to be its physiological response to changing abiotic variables over its range. The range of a species often corresponds to the geographic extent of temperature regimes the organism can physiologically tolerate. Many species have very distinct life history stages that may exhibit different responses to environmental factors. In this study we emphasized the critical role of the haploid microscopic stage (gametophyte) of the life cycle to explain the difference of edge distribution of two related kelp species. Lessonia nigrescens was recently identified as two cryptic species occurring in parapatry along the Chilean coast: one located north and the other south of a biogeographic boundary at latitude 29–30°S. Six life history traits from microscopic stages were identified and estimated under five treatments of temperature in eight locations distributed along the Chilean coast in order to (1) estimate the role of temperature in the present distribution of the two cryptic L. nigrescens species, (2) compare marginal populations to central populations of the two cryptic species. In addition, we created a periodic matrix model to estimate the population growth rate (λ) at the five temperature treatments. Differential tolerance to temperature was demonstrated between 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 exhibited different life history strategies with a shorter haploid phase in the Northern species contrasted with considerable vegetative growth in the Southern species haploid stage. These results provide strong ecological evidence for the differentiation process of the two cryptic species and show local adaptation of the life cycle at the range limits of the distribution. Ecological and evolutionary implications of these findings are discussed.
Highlights
Understanding the mechanisms that limit geographical distributions of species has long been a key question in both ecology and evolutionary biology [1,2] and it is generally accepted that multiple causes can be interacting
It has been hypothesized that this survival could have been the result of local adaptation to high temperatures [36,37]. Using these two cryptic species of the Lessonia, we aimed to test the following hypotheses regarding the tolerance to temperature stress of microscopic stages: (i) the Northern species is expected to be more tolerant to high temperature than the Southern species, and (ii) local adaptation within each species is expected as a differential thermal tolerance among populations across geographic range
In contrast to the Northern species, the Southern cold-waters species is not able to survive or to reproduce at high temperatures. These results suggest that, as expected, the Northern species is tolerant to higher temperatures than the Southern one, and that the geographic distribution of these two species seems to be related to the environmental conditions
Summary
Understanding the mechanisms that limit geographical distributions of species has long been a key question in both ecology and evolutionary biology [1,2] and it is generally accepted that multiple causes (both biotic and abiotic) can be interacting. To study the mechanisms that constrain a species’ distribution, a highly informative zone is the edge of the range itself [10,11], to study evolutionary processes. Marginal populations are expected to be genetically deprived, leading to a higher sensitivity to environmental changes in comparison with central populations. Marginal populations may be a place where local adaptation occurs, due to the particular environmental conditions, this process depends on the relative genetic isolation of the marginal populations and on the species’ dispersal capacity
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