Essay: Chromosome number

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Chromosome number and the drivers of its variation were examined in 6 major angiosperm clades of Italian endemic vascular plants. As chromosome number can be employed as a surrogate of recombination rate, we investigated whether low chromosome numbers are favoured in individuals living in unstable environments ensuring immediate fitness.
 
We tested our hypothesis by relating chromosome numbers with other plant traits and environmental variables. To this end, we used a phylogenetic comparative approach that estimates phylogenetic inertia and adaptation in chromosome number based on an Ornstein-Uhlenbeck process.

Phylogenetic effects in chromosome number varied among examined clades, but were generally high. Chromosome number resulted poorly related to large scale climatic conditions, while a stronger relation with categorical variables was found. Specifically, open, disturbed, drought-prone habitats selected for low chromosome numbers, while perennial herbs, and especially woody plants, living in shaded, stable environments were associated with high chromosome numbers.

Altogether, our findings confirm our expectations and we argue that environmental stability favour higher recombination rates with respect to unstable environments. In addition, by comparing results of models testing for evolvability of 2n and of x, we provided insight into the ecological significance of polyploidy.

We investigated whether the evolution towards optima of diploid (2n) and basic (x) chromosome numbers is influenced by climatic variables (continuous predictors), habitat characteristics or plant traits (categorical predictors) within different angiosperm clades.

To this end, we used the phylogenetic comparative method implemented in the R program SLOUCH, conceived to study adaptive evolution of a trait along a phylogenetic tree (Hansen et al., 2008). The method assumes that the response trait evolves as if by an Ornstein–Uhlenbeck model of adaptive evolution, towards a ‘primary’ optimum θ, defined as the optimal state that species will approach in a given niche when ancestral constraints have disappeared (Hansen, 1997).

Phylogenetic trees are scaled to 1.0 total length (from the root to the tip in the ultrametric tree) to facilitate the interpretation of parameter estimates. The two main parameters returned by the model are the phylogenetic half-life (t1/2) and the stationary variance (vy). Phylogenetic half-life indicates the time it takes for half the ancestral influence on a trait to evolve towards the predicted optimal phenotype (Hansen, 1997). A half-life above zero indicates that adaptation is not instantaneous, while when t1/2 = 0 means that there is no evolutionary lag. The stationary variance is the stochastic component of the model and can be interpreted as evolutionary changes in the response trait induced by genetic drift or unmeasured selective forces.

Phylogenetic half-life in a model that only includes the intercept is a measure of the phylogenetic effect in the response trait. In such a model, a half-life = zero means that the response variable is not phylogenetically structured, while a half-life > 0 indicates that there is an influence of phylogeny on the data; when the half-life shows high values, this can attributed to an underlying continuous Brownian motion process.

The intercept-only model is contrasted with a model that also includes a predictor variable.

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