Evolution of postzygotic reproductive isolation in galliform birds: analysis of first and second hybrid generations and backcrosses

The process of speciation is a crucial aspect of evolutionary biology. In this study, we analysed the patterns of evolution of postzygotic reproductive isolation in Galliformes using information on hybridization and genetic distance among species. Four main patterns arose: (1) hybrid inviability and sterility in F₁ hybrids increase as species diverge; (2) the presence of geographical overlap does not affect the evolution of postzygotic isolation; (3) the galliforms follow Haldane's rule; (4) hybrid inviability is higher in F₂ than in F₁ hybrids, but does not appear to be increased in the backcrosses. This study contributes to the growing evidence suggesting that the patterns of evolution of postzygotic isolation and the process of speciation are shared among avian groups (and animals in general). In particular, our results support the notion of F₂ hybrid inviability as being key for the maintenance of species genetic integrity when prezygotic isolation barriers are overcome in closely related species, in which postzygotic isolation in the F₁ hybrid might still not be fully developed. To the contrary, hybrids from backcrosses did not show serious inviability problems (at least not more than F₁ hybrids), demonstrating that they could generate gene flow among bird species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 528–542.     

Structural study of the ovary,oogenesis and brooding in Tonicia lebruni (Polyplacophora Chitonidae) from Patagonia

Tonicia lebruni, a common, lower intertidal and subtidal chiton inhabiting Patagonian rocky shores, is a gonochoristic iteroparous species producing large eggs (≈ 400 μm in diameter), which are fertilized and brooded within the pallial groves until released as juveniles. A free larval stage is absent, despite this, T. lebruni is widely distributed along the south‐western Atlantic. At Puerto Deseado, T. lebruni has a marked seasonality in the reproductive cycle, reproducing only once a year. The reproductive period is quite short and defined in time: spawning and brooding take place during the late austral winter and beginning of spring. Recovery of the female gonad starts very soon after spawning. Oogenesis takes about 10–11 months for completion. Brood size is correlated with length of maternal individual. The number of embryos per brood varied between 785 and 5945. Extensive resorption of abortive eggs is viewed as related to limitation of space available for brooding. The egg hull is formed by a large number of minute pentagonal or hexagonal plates each one bearing a short spine bent onto the egg surface. The morphology and the surface of the hull could contribute to the cohesiveness of the brooded egg mass within the pallial grooves.     

Differential effect of shade,water and soil type on emergence and early survival of three dominant species of the Atacama Desert

Understanding the regeneration niche of species may allow us to gain insight into how communities are structured. In deserts, the regeneration niche is usually related to spaces beneath shrubs where shade cast by shrubs creates microenvironments that benefit seedlings and where even small amounts of rain may favour germination and establishment. Shade and water may also interact with different types of soils. However, species may have different requirements for germination and seedling survival. We could expect that shrub species with different drought tolerances exhibit different responses to the combination of these factors. We ask if responses of dominant species of the Atacama Desert to abiotic factors (shade, water and soil type) are related to their drought tolerance, a topic not exhaustively explored in shrubs growing in true deserts. We conducted two factorial experiments. The first one was designed to evaluate how shade (microhabitat) in combination with water may affect germination (emergence) and early survival. In the second experiment, we assessed the influence of shade in relation to soil type. Each species responded distinctively to the three variables under study, but in general, their emergence responses were more influenced by water (more water, greater emergence) than by microhabitat or soil type. Survival was influenced both by microhabitat and by water and was higher under shade and abundant water. Soil type affected only one of our species in terms of emergence. Species responses in general depended on their tolerance to stress. In one species, there was indication of a seed–seedling conflict. Our results show similar species responses to environmental constraints but also more or less unique responses that are related to their tolerance to drought and which may ultimately permit species coexistence. We found that shade may not be important for germination but may be crucial for survival in dry years.     

Expression of Arabidopsis Bax Inhibitor‐1 in transgenic sugarcane confers drought tolerance

The sustainability of global crop production is critically dependent on improving tolerance of crop plants to various types of environmental stress. Thus, identification of genes that confer stress tolerance in crops has become a top priority especially in view of expected changes in global climatic patterns. Drought stress is one of the abiotic stresses that can result in dramatic loss of crop productivity. In this work, we show that transgenic expression of a highly conserved cell death suppressor, Bax Inhibitor‐1 from Arabidopsis thaliana (AtBI‐1), can confer increased tolerance of sugarcane plants to long‐term (>20 days) water stress conditions. This robust trait is correlated with an increased tolerance of the transgenic sugarcane plants, especially in the roots, to induction of endoplasmic reticulum (ER) stress by the protein glycosylation inhibitor tunicamycin. Our findings suggest that suppression of ER stress in C₄ grasses, which include important crops such as sorghum and maize, can be an effective means of conferring improved tolerance to long‐term water deficit. This result could potentially lead to improved resilience and yield of major crops in the world.     

Quantifying long‐term recurrence in planktonic microbial eukaryotes

How much temporal recurrence is present in microbial assemblages is still an unanswered ecological question. Even though marked seasonal changes have been reported for whole microbial communities, less is known on the dynamics and seasonality of individual taxa. Here, we aim at understanding microbial recurrence at three different levels: community, taxonomic group and operational taxonomic units (OTUs). For that, we focused on a model microbial eukaryotic community populating a long‐term marine microbial observatory using 18S rRNA gene data from two organismal size fractions: the picoplankton (0.2–3 µm) and the nanoplankton (3–20 µm). We have developed an index to quantify recurrence in particular taxa. We found that community structure oscillated systematically between two main configurations corresponding to winter and summer over the 10 years studied. A few taxonomic groups such as Mamiellophyceae or MALV‐III presented clear recurrence (i.e., seasonality), whereas 13%–19% of the OTUs in both size fractions, accounting for ~40% of the relative abundance, featured recurrent dynamics. Altogether, our work links long‐term whole community dynamics with that of individual OTUs and taxonomic groups, indicating that recurrent and non‐recurrent changes characterize the dynamics of microbial assemblages.     

Impact of grazing,resource availability and light on prokaryotic growth and diversity in the oligotrophic surface global ocean

The impact of grazing, resource competition and light on prokaryotic growth and taxonomic composition in subtropical and tropical surface waters were studied through 10 microcosm experiments conducted between 30°N and 30°S in the Atlantic, Pacific and Indian oceans. Under natural sunlight conditions, significant changes in taxonomic composition were only observed after the reduction of grazing by sample filtration in combination with a decrease in resource competition by sample dilution. Sunlight exposure significantly reduced prokaryote growth (11 ± 6%) and community richness (14 ± 4%) compared to continuous darkness but did not significantly change community composition. The largest growth inhibition after sunlight exposure occurred at locations showing deep mixed layers. The reduction of grazing had an expected and significant positive effect on growth, but caused a significant decrease in community richness (16 ± 6%), suggesting that the coexistence of many different OTUs is partly promoted by the presence of predators. Dilution of the grazer-free prokaryotic community significantly enhanced growth at the level of community, but consistently and sharply reduced the abundance of Prochlorococcus and SAR11 populations. The decline of these oligotrophic bacterial taxa following an increase in resource availability is consistent with their high specialization for exploiting the limited resources available in the oligotrophic warm ocean.     

Comparative polytene chromosome maps of <Emphasis Type="Italic">D. montana</Emphasis> and <Emphasis Type="Italic">D. virilis</Emphasis>

Chromosomal inversion polymorphism was characterized in Finnish Drosophila montana populations. A total of 14 polymorphic inversions were observed in Finnish D. montana of which nine had not been described before. The number of polymorphic inversions in each chromosome was not significantly different from that expected, assuming equal chance of occurrence in the euchromatic genome. There was, however, no correlation between the number of polymorphic inversions and that of fixed inversions in each chromosome. Therefore, a simple neutral model does not explain the evolutionary dynamics of inversions. Furthermore, in contrast to results obtained by others, no significant correlation was found between the two transposable elements (TEs) Penelope and Ulysses and inversion breakpoints in D. montana. This result suggests that these TEs were not involved in the creation of the polymorphic inversions seen in D. montana. A comparative analysis of D. montana and Drosophila virilis polytene chromosomes 4 and 5 was performed with D. virilis bacteriophage P1 clones, thus completing the comparative studies of the two species.     

Xeroderma pigmentosum complementation group A protein is driven to nucleotide excision repair sites by the electrostatic potential of distorted DNA

The presumed DNA-binding cleft of xeroderma pigmentosum group A (XPA) protein, a key regulatory subunit of the eukaryotic nucleotide excision repair complex, displays a distinctive array of 6 positively charged amino acid side chains. Here, the molecular function of these closely spaced electropositive residues has been tested by systematic site-directed mutagenesis. After the introduction of single amino acid substitutions, the mutants were probed for protein-DNA interactions in electrophoretic mobility shift and photochemical crosslinking assays. This analysis led to the identification of a critical hot-spot for DNA substrate recognition composed of two neighboring lysines at codons 141 and 179 of the human XPA sequence. The replacement of other basic side chains in the DNA interaction domain conferred more moderate defects of substrate binding. When the function of XPA was tested as a fusion product with either mCherry or green-fluorescent protein, a glutamate substitution of one of the positively charged residues at positions 141 and 179 was sufficient to decrease DNA repair activity in human fibroblasts. Thus, the removal of a single cationic side chain abolished DNA-binding activity and significant excision repair defects could be induced by single charge inversions on the XPA surface, indicating that this molecular sensor participates in substrate recognition by monitoring the electrostatic potential of distorted DNA repair sites.