Fine‐scale geographic patterns of gene flow and reproductive character displacement in Drosophila subquinaria and Drosophila recens

When two species are incompletely isolated, strengthening premating isolation barriers in response to the production of low fitness hybrids may complete the speciation process. Here, we use the sister species Drosophila subquinaria and Drosophila recens to study the conditions under which this reinforcement of species boundaries occurs in natural populations. We first extend the region of known sympatry between these species, and then we conduct a fine‐scale geographic survey of mate discrimination coupled with estimates of gene flow within and admixture between species. Within D. subquinaria, reinforcement is extremely effective: we find variation in mate discrimination both against D. recens males and against conspecific allopatric males on the scale of a few kilometres and in the face of gene flow both from conspecific populations and introgression from D. recens. In D. recens, we do not find evidence for increased mate discrimination in sympatry, even where D. recens is rare, consistent with substantial gene flow throughout the species’ range. Finally, we find that introgression between species is asymmetric, with more from D. recens into D. subquinaria than vice versa. Within each species, admixture is highest in the geographic region where it is rare relative to the other species, suggesting that when hybrids are produced they are of low fitness. In sum, reinforcement within D. subquinaria is effective at maintaining species boundaries, but even when reinforcing selection is strong it may not always result in a pattern of strong reproductive character displacement due to variation in the frequency of hybridization and gene flow from neighbouring populations.     

Variation in management of small invasive breast cancers detected on screening in the former south east Thames region: observational study.

OBJECTIVE: To examine the variation in surgical and adjuvant treatment of breast cancer of known histology and detected on screening in a large cohort of patients treated by the surgeons of a health region. DESIGN: Part prospective, part retrospective observational study using the databases of a region''s breast screening programme and of the cancer registry. SETTING: The former South East Thames region. SUBJECTS: 600 women aged 49-79 who presented during 1991-2 with invasive breast cancer up to 20 mm in diameter that had been detected on screening. These patients were treated by 35 surgeons. MAIN OUTCOME MEASURES: Mastectomy rate by surgeon and the use of adjuvant treatment (radiotherapy, tamoxifen, and chemotherapy) were compared with risk factors, tumour grade, resection margins, and axillary node status. RESULTS: The mastectomy rate varied between nil and 80%, although the numbers at these extremes were small (0/13 v 8/10). Surgeons operating on more than 20 such cases had a lower mastectomy rate (15%) than surgeons treating fewer cases (23%), but this difference was confounded by variation in casemix. There were also wide variations in mastectomy rates and in axillary sampling rates that were independent of casemix or caseload. There was broad agreement on the use of adjuvant tamoxifen (94%), but few patients received chemotherapy (2.5%). 78 patients (19%) did not receive radiotherapy, including 51 out of 317 patients with unfavourable tumours, and 26 patients did not receive tamoxifen. Whether the patient received adjuvant treatment was more dependent on referral by the surgeon than the risk factors for local recurrence and was independent of caseload. CONCLUSION: Mastectomy rates for similar tumours vary widely by surgeon independently of casemix or caseload, but surgeons with a higher caseload tend to have a lower mastectomy rate. Omission of postoperative radiotherapy or tamoxifen after conservative treatment is not related to risk factors for local recurrence or caseload. Confidential feedback of treatment profiles to individual surgeons has been used, but when benefit has been established treatment should be guided by evidence based protocol.     

Population density of the Réunion Grey White-eye Zosterops borbonicus within the summit ecosystems of Réunion,Mascarene Islands

Assessing population density is crucial for studying the ecology and evolutionary biology of species as well as for conservation purposes. Here we used point count methods to infer population density in a single-island endemic passerine bird, the Réunion Grey White-eye Zosterops borbonicus, that displays striking evidence of differentiation at a small spatial scale. Population density was estimated at 5.17 birds ha^?1 (CL 4.85–5.50), a value somewhat higher than previously believed. This estimation provides the first detailed estimation of bird population density in the vulnerable summit ecosystems of Réunion and will possibly allow a better understanding of the evolutionary causes of this plumage colour variation.     

From near extinction to diversification by means of a shift in pollination mechanism in the gymnosperm relict Ephedra (Ephedraceae,Gnetales)

Pollination in gymnosperms is usually accomplished by means of wind, but some groups are insect‐pollinated. We show that wind and insect pollination occur in the morphologically uniform genus Ephedra (Gnetales). Based on field experiments over several years, we demonstrate distinct differences between two Ephedra species that grow in sympatry in Greece in pollen dispersal and clump formation, insect visitations and embryo formation when insects are denied access to cones. Ephedra distachya, nested in the core clade of Ephedra, is anemophilous, which is probably the prevailing state in Ephedra. Ephedra foeminea, sister to the remaining species of the genus, is entomophilous and pollinated by a range of diurnal and nocturnal insects. The generalist entomophilous system of E. foeminea, with distinct but infrequent insect visitations, is in many respects similar to that reported for Gnetum and Welwitschia and appears ancestral in Gnetales. The Ephedra lineage is well documented already from the Early Cretaceous, but the diversity declined dramatically during the Late Cretaceous, possibly to near extinction around the Cretaceous–Palaeogene boundary. The clade imbalance between insect‐ and wind‐pollinated lineages is larger than expected by chance and the shift in pollination mode may explain why Ephedra escaped extinction and began to diversify again.     

Lesions to right posterior parietal cortex impair visual depth perception from disparity but not motion cues

The posterior parietal cortex (PPC) is understood to be active when observers perceive three-dimensional (3D) structure. However, it is not clear how central this activity is in the construction of 3D spatial representations. Here, we examine whether PPC is essential for two aspects of visual depth perception by testing patients with lesions affecting this region. First, we measured subjects'' ability to discriminate depth structure in various 3D surfaces and objects using binocular disparity. Patients with lesions to right PPC (N = 3) exhibited marked perceptual deficits on these tasks, whereas those with left hemisphere lesions (N = 2) were able to reliably discriminate depth as accurately as control subjects. Second, we presented an ambiguous 3D stimulus defined by structure from motion to determine whether PPC lesions influence the rate of bistable perceptual alternations. Patients'' percept durations for the 3D stimulus were generally within a normal range, although the two patients with bilateral PPC lesions showed the fastest perceptual alternation rates in our sample. Intermittent stimulus presentation reduced the reversal rate similarly across subjects. Together, the results suggest that PPC plays a causal role in both inferring and maintaining the perception of 3D structure with stereopsis supported primarily by the right hemisphere, but do not lend support to the view that PPC is a critical contributor to bistable perceptual alternations.This article is part of the themed issue ‘Vision in our three-dimensional world’.     

Cytogenetic investigations in a family with ataxia telangiectasia

Summary Cytogenetic findings on a family with ataxia telangiectasia (A-T) in which three of four sibs were affected are described. The affected individuals had approximately twice the level of spontaneous chromosome breakage of a normla control, while the parents and the normal sib had no significant increase. Lymphocytes from all three A-T homozygotes showed specific stable chromosomal rearrangements involving chromosomes 7 and 14. All of these abnormalities involved breakage at the usual four sites associated with A-T (7p14, 7q35, 14q12, and 14q32). Two rearrangements detected in the eldest and most severely affected patient were clones, one of which [t(14;14)(p11;q12)] is not commonly found in A-T cells. No chromosomal rearrangements were encountered in lymphocytes from the control, the parents, or the normal sib. Lymphocytes from the A-T patients also were found to be 7–11 times more sensitive to the induction of chromatid aberrations by X-irradiation than control cells. Lymphocytes from the parents and normal sib showed a moderately increased frequency of X-ray induced aberrations compared with that of the control.     

Molecular Studies of R Factor Compatibility Groups

Molecular studies of R factors of six groups, F_II, I₁, I₂, N, B, and H, defined on the basis of compatibility, support the conclusions drawn from genetic studies. In general, R factors of a given compatibility group are similar in size. Deoxyribonucleic acid (DNA) reassociation occurs freely between members of the same group but is minimal between heterologous groups. An exception to this was found in group H, of which one factor showed minimal homology with the remaining plasmids of the group. A further exception was found with groups I₁ and B, which, although genetically distinct, show between 18 and 28% of DNA homology. Groups I₁ and I₂ are molecularly distinct, despite the fact that they both stimulate the synthesis of I-fimbriae.     

An hypothesis on the binding of an amphipathic, alpha helical sequence in Ii to the desetope of class II antigens

When we investigated the hypothesis that amphipathic alpha helical peptides digested from foreign antigen bind to class II major histocompatability complex (MHC) molecules' binding site (desetope) for foreign antigen to be presented to T cell receptors, we found such an extended amphipathic helix in Ii. This amphipathic helix was hypothesized to bind Ii to class II MHC antigens until release in endosomes containing digested foreign antigen. Then these amphipathic Ii polypeptides might polymerize so as not to compete with foreign antigen for binding to class II MHC molecules. Various structural models were consistent with these views and led to the suggestion of specific forms of polymeric interaction.