Male-male competition and repeated evolution of terrestrial breeding in Atlantic Coastal Forest frogs.

Terrestrial breeding is a derived condition in frogs, with multiple transitions from an aquatic ancestor. Shifts in reproductive mode often involve changes in habitat use, and these are typically associated with diversification in body plans, with repeated transitions imposing similar selective pressures. We examine the diversification of reproductive modes, male and female body sizes, and sexual size dimorphism (SSD) in the Neotropical frog genera Cycloramphus and Zachaenus, both endemic to the Atlantic rainforest of Brazil. Species in this clade either breed in rocky streams (saxicolous) or in terrestrial environments, allowing us to investigate reproductive habitat shifts. We constructed a multilocus molecular phylogeny and inferred evolutionary histories of reproductive habitats, body sizes, and SSD. The common ancestor was small, saxicolous, and had low SSD. Terrestrial breeding evolved independently three times and we found a significant association between reproductive habitat and SSD, with shifts to terrestrial breeding evolving in correlation with decreases in male body size, but not female body size. Terrestrial breeding increases the availability of breeding sites and results in concealment of amplexus, egg-laying, and parental care, therefore reducing male-male competition at all stages of reproduction. We conclude that correlated evolution of terrestrial reproduction and small males is due to release from intense male-male competition that is typical of exposed saxicolous breeding.

Narrow thermal tolerance and low dispersal drive higher speciation in tropical mountains.

Species richness is greatest in the tropics, and much of this diversity is concentrated in mountains. Janzen proposed that reduced seasonal temperature variation selects for narrower thermal tolerances and limited dispersal along tropical elevation gradients [Janzen DH (1967) Am Nat 101:233-249]. These locally adapted traits should, in turn, promote reproductive isolation and higher speciation rates in tropical mountains compared with temperate ones. Here, we show that tropical and temperate montane stream insects have diverged in thermal tolerance and dispersal capacity, two key traits that are drivers of isolation in montane populations. Tropical species in each of three insect clades have markedly narrower thermal tolerances and lower dispersal than temperate species, resulting in significantly greater population divergence, higher cryptic diversity, higher tropical speciation rates, and greater accumulation of species over time. Our study also indicates that tropical montane species, with narrower thermal tolerance and reduced dispersal ability, will be especially vulnerable to rapid climate change.

Ecotypes of an ecologically dominant prairie grass (Andropogon gerardii) exhibit genetic divergence across the U.S. Midwest grasslands' environmental gradient.

Big bluestem (Andropogon gerardii) is an ecologically dominant grass with wide distribution across the environmental gradient of U.S. Midwest grasslands. This system offers an ideal natural laboratory to study population divergence and adaptation in spatially varying climates. Objectives were to: (i) characterize neutral genetic diversity and structure within and among three regional ecotypes derived from 11 prairies across the U.S. Midwest environmental gradient, (ii) distinguish between the relative roles of isolation by distance (IBD) vs. isolation by environment (IBE) on ecotype divergence, (iii) identify outlier loci under selection and (iv) assess the association between outlier loci and climate. Using two primer sets, we genotyped 378 plants at 384 polymorphic AFLP loci across regional ecotypes from central and eastern Kansas and Illinois. Neighbour-joining tree and PCoA revealed strong genetic differentiation between Kansas and Illinois ecotypes, which was better explained by IBE than IBD. We found high genetic variability within prairies (80%) and even fragmented Illinois prairies, surprisingly, contained high within-prairie genetic diversity (92%). Using Bayenv2, 14 top-ranked outlier loci among ecotypes were associated with temperature and precipitation variables. Six of seven BayeScanFST outliers were in common with Bayenv2 outliers. High genetic diversity may enable big bluestem populations to better withstand changing climates; however, population divergence supports the use of local ecotypes in grassland restoration. Knowledge of genetic variation in this ecological dominant and other grassland species will be critical to understanding grassland response and restoration challenges in the face of a changing climate.

The daughter four-membered microtubule rootlet determines anterior-posterior positioning of the eyespot in Chlamydomonas reinhardtii.

The characteristic geometry of the unicellular chlorophyte Chlamydomonas reinhardtii has contributed to its adoption as a model system for cellular asymmetry and organelle positioning. The eyespot, a photosensitive organelle, is localized asymmetrically in the cell at a precisely defined position relative to the flagella and cytoskeletal microtubule rootlets. We have isolated a mutant, named pey1 for posterior eyespot, with variable microtubule rootlet lengths. The length of the acetylated daughter four-membered (D4) microtubule rootlet correlates with the position of the eyespot, which appears in a posterior position in the majority of cells. The correlation of rootlet length with eyespot positioning was also observed in the cmu1 mutant, which has longer acetylated microtubules, and the mlt1 mutant, in which the rootlet microtubules are shorter. Observation of eyespot positioning after depolymerization of rootlet microtubules indicated that eyespot position is fixed early in eyespot development and becomes independent of the rootlet. Our data demonstrate that the length of the D4 rootlet is the major determinant of eyespot positioning on the anterior-posterior axis and are suggestive that the gene product of the PEY1 locus is a novel regulator of acetylated microtubule length.