Resource defence in female pollinating fig wasps: two's a contest, three's a crowd

Female pollinating fig wasp (foundress) reproduction is often reduced when multiple foundresses oviposit in figs, owing to oviposition site limitation or the extra time spent searching for unused sites. In species where foundresses cannot re-emerge from figs (meaning that all reproduction takes place in a single fig), they might therefore be expected to defend sites from competitors. For the first time, we explicitly documented defence by foundresses. Larger Platyscapa awekei foundresses (pollinators of Ficus salicifolia) in two-foundress figs prevented smaller foundresses from ovipositing by holding them in their mandibles while ovipositing themselves, and deposited the same number of eggs as when ovipositing in isolation. Smaller foundresses deposited around 30% fewer eggs. The onset of defence in two-foundress figs depended on larger foundress body size, implying that they made decisions based on their own size relative to the population mean (i.e. the average probability of being successful). Defence was less common in three- than in two-foundress figs, probably because the presence of a third competitor reduced the benefits accrued. We discuss why foundresses use such tactics, and identify other species in which defence may occur. Qualitative behavioural observations and comparative morphology suggest that defence is common, occurring in four pollinator genera. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.      

Alternative splicing--when two's a crowd

Various mechanisms have been proposed to explain mutually exclusive splicing of pairs of exons. A paper in this issue of Cell (Graveley, 2005) provides a fascinating insight into the perplexing question of how only one exon at a time is chosen from an array of 48 exons in the Drosophila Dscam gene.     

Two's company, three's a crowd: experimental evaluation of the evolutionary maintenance of trioecy in Mercurialis annua (Euphorbiaceae)

Trioecy is an uncommon sexual system in which males, females, and hermaphrodites co-occur as three clearly different gender classes. The evolutionary stability of trioecy is unclear, but would depend on factors such as hermaphroditic sex allocation and rates of outcrossing vs. selfing. Here, trioecious populations of Mercurialis annua are described for the first time. We examined the frequencies of females, males and hermaphrodites across ten natural populations and evaluated the association between the frequency of females and plant densities. Previous studies have shown that selfing rates in this species are density-dependent and are reduced in the presence of males, which produce substantially more pollen than hermaphrodites. Accordingly, we examined the evolutionary stability of trioecy using an experiment in which we (a) indirectly manipulated selfing rates by altering plant densities and the frequency of males in a fully factorial manner across 20 experimental plots and (b) examined the effect of these manipulations on the frequency of the three sex phenotypes in the next generation of plants. In the parental generation, we measured the seed and pollen allocations of hermaphrodites and compared them with allocations by unisexual plants. In natural populations, females occurred at higher frequencies in denser patches, a finding consistent with our expectations. Under our experimental conditions, however, no combination of plant densities and male frequencies was associated with increased frequencies of females. Our results suggest that the factors that regulate female frequencies in trioecious populations of M. annua are independent of those regulating male frequencies (density), and that the stable co-existence of all three sex phenotypes within populations is unlikely.     

Two's company,three's a crowd: new insights on spruce budworm species boundaries using genotyping‐by‐sequencing in an integrative species assessment (Lepidoptera: Tortricidae)

Species delimitation requires an assessment of varied traits that can contribute to reproductive isolation, as well as of the permanence of evolutionary differentiation among closely related lineages. Integrative taxonomy, including the combination of genome‐wide molecular data with ecological data, offers an effective approach to this issue. We use genotyping‐by‐sequencing together with a review of ecological divergence to assess the traditionally recognized species status of three closely related members of the spruce budworm species complex, Choristoneura fumiferana (Clemens), C. occidentalis Freeman (=C. freemani Razowski) and C. biennis Freeman, each of which is a major defoliator of conifer forests. We sampled a broad region of overlap between these three taxa in Alberta and British Columbia (Canada) where potential for gene flow provides a strong test of the durability of divergence among lineages. A total of 2218 single nucleotide polymorphisms (SNPs) were assayed, and patterns of differentiation were evaluated under the biological, ecological, genotypic cluster and phylogenetic species concepts. Choristoneura fumiferana was genetically distinct with substantial barriers to genetic exchange with C. occidentalis and C. biennis. Conversely, divergence between C. occidentalis and C. biennis was limited to a small subset of outlier loci and was within the range observed within any one of the taxa. Considering both population genetic and ecological patterns of divergence, C. fumiferana should continue to be recognized as a distinct species, and C. biennis ( syn.n. ) should be treated as a subspecies (C. occidentalis biennis Freeman, 1967) of C. occidentalis, thereby automatically establishing the nominate name C. occidentalis occidentalis Freeman, 1967 for univoltine populations of this species.     

MAPKAP kinase-2: three's company at the G(2) checkpoint

Elegant studies in fission yeast by and in mammalian cells by offer new insights into the mechanism through which stress-induced p38 activation inhibits mitotic entry in eukaryotic cells.     

Mosaic autosomal trisomy in cultures from spontaneous abortions.

In a consecutive series of 592 karyotyped spontaneous abortions, ten of 103 autosomal trisomies were mosaic, with a normal cell line also present. The frequency of mosaicism (10%) is much higher than that reported in Down syndrome, but similar to that reported in amniotic fluid cultures and in induced abortions. The most likely explanations for this discrepancy are (1) previous underestimation of mosaicism in live births or (2) mosaicism which is often restricted to extraembryonic fetal tissue.     

Partial distal trisomy 3p. A partial autosomal trisomy without major dysmorphic features

Whereas in the great majority of autosomal duplications/deficiencies a clinically recognizable dysmorphic syndrome is present, distal 3p duplication is not associated with major dysmorphic signs. We present the clinical data and molecular cytogenetic findings in two non-related patients. Diagnosis was made in a female child at the age of 5 months because of psychomotor retardation and slight dysmorphism. She also presented hydronefrosis and develops no speech at the age of almost 4 years. Her partial trisomy is the result of an inverted duplication 3p22-->3pter (dup(3)(pter-->p26::p22(p26::p26-->ter)). An adult woman was diagnosed at the of 80 years only on the basis of mental retardation and poor speech development, but without evident dysmorphism. In this patient the partial 3p trisomy is the unbalanced product of a 3p/17p translocation: t(3;7)(p253;p133).     

Synaptonemal complex analysis of an autosomal trisomy in the horse.

Synaptonemal complex analysis by electron microscopy of a trisomy 28 in a male horse demonstrated a trivalent or a bivalent plus a univalent in primary spermatocytes. Two of the chromosomes making up the trivalent were, most often, completely paired with each other and only partially paired or associated with the third one. Half of the spermatocytes analysed demonstrated heterologous pairing or association between the free axis of the trivalent and the sex bivalent. The pairings remained, to a large extent, into diakinesis-metaphase I. In most pachytene cells one autosomal bivalent showed proximal asynapsis and paired often, heterologously, with the trivalent or the sex bivalent. The horse demonstrated azoospermy, which was due, at least in part, to degeneration at both the spermatocyte and spermatid levels.     

The relationship between maternal age and chromosome size in autosomal trisomy.

The pattern of maternal age-specific incidence of autosomal trisomy in spontaneous abortions was examined for each chromosome for which a sufficient number of trisomies was observed. This included chromosomes 2, 4, 7-10, 13-16, 18, and 20-22. The rate of increase after age 30 for each of the small chromosomes (groups D-G) was similar, with the exception of chromosome 16, which showed a significantly shallower rate. The C group chromosomes tended to have an intermediate rate of increase after age 30, with the exception of chromosome 7, which had a pattern similar to the smaller chromosomes. The larger chromosomes (2 and 4) had the smallest rate of increase. There was a significant relationship between chromosome size and rate of increase after age 30 (after excluding chromosome 16), but not with rate of increase before age 30. The results suggest that autosomal trisomies may be of heterogeneous origin, with a maternal age-related factor associated with chromosome size and other sources unrelated to chromosome size. Additional evidence for and against this hypothesis is discussed.     

Familial translocation 22/Y and partial autosomal trisomy in a young girl

Report on a translocation t(22;Y)(q12;p 13) with conservation of the NOR in normal members from 2 generations of a family. The proposita has in addition a small autosomal duplication, probably (1)(q44-ter) which could explain her mental deficiency.     

Is two company or a crowd: How does conspecific density affect the small-scale dispersal of a weed biocontrol agent?

To predict the growth and spread of an insect population introduced for the biological control of weeds, one must first understand the factors affecting the movement of individuals in the population. The purpose of this study was to determine how the dispersal rate of Aphthona lacertosa (Rosenhauer) (Chrysomelidae) was affected by conspecific density and by the characteristics of leafy spurge (Euphorbia esula L.: Euphorbiaceae) in patches where these beetles feed. In 2002 in Manitoba and in 2003 in Alberta, Canada, between 200 and 2500 insects were released in small patches (<10 m²) of spurge. The number and location of beetles within patches was monitored over subsequent days. In 1 m² plots within patches, spurge ramet density, the proportions of vegetative and reproductive ramets, and ramet height were measured. In both years, beetle movement within patches and emigration from patches, was not affected by conspecific density. In Manitoba in 2002, beetles aggregated non-randomly on either vegetative or reproductive ramets within plots, but plot characteristics were not related to the formation of aggregations. In Alberta in 2003, plots in which beetles aggregated had significantly higher spurge density but did not differ in either the proportion of vegetative ramets or in the amount of non-spurge vegetation. These results suggest that density-dependent dispersal does not limit the population's ability to reach densities up to 2500 beetles/m².     

Mouse trisomy 16 as an animal model of human trisomy 21 (Down syndrome): production of viable trisomy 16 diploid mouse chimeras

We have previously proposed that mice trisomic for chromosome 16 will provide an animal model of human trisomy 21 (Down syndrome). However, the value of this model is limited to some extent because trisomy 16 mouse fetuses do not survive as live-born animals. Therefore, in an effort to produce viable mice with cells trisomic for chromosome 16, we have used an aggregation technique to generate trisomy 16 diploid (Ts 16 2n) chimeras. A total of 79 chimeric mice were produced, 11 of which were Ts 16 2n chimeras. Seven of these Ts 16 2n mice were analyzed as fetuses, just prior to birth, and 4 were analyzed as live-born animals. Unlike nonchimeric Ts 16 mouse fetuses which die shortly before birth with edema, congenital heart disease, and thymic and splenic hypoplasia, all but 1 of the Ts 16 2n animals were viable and phenotypically normal. The oldest of the live-born Ts 16 2n chimeras was 12 months old at the time of necropsy. Ts 16 cells, identified by coat color, enzyme marker, and/or karyotype analyses, comprised 50-60% of the brain, heart, lung, liver, and kidney in the 7 Ts 16 2n chimeric fetuses and 30-40% of these organs in the 4 live-born Ts 16 2n animals. Ts 16 cells comprised an average of 40% of the thymus and 80% of the spleen in the Ts 16 2n chimeras analyzed as fetuses, with no evidence of thymic or splenic hypoplasia. However, we observed a marked deficiency to Ts 16 cells in the blood, spleen, thymus, and bone marrow of live-born Ts 16 2n chimeras as compared to 2n 2n controls. These results demonstrate that although the Ts 16 2n chimeras were, with one exception, viable and phenotypically normal, each animal contained a significant proportion of trisomic cells in a variety of tissues, including the brain. Furthermore, our results suggest that although the abnormal development of Ts 16 thymus and spleen cells observed in Ts 16 fetuses is largely corrected in Ts 16 2n fetuses, Ts 16 erythroid and lymphoid cells have a severe proliferative disadvantage as compared to diploid cells in older live-born Ts 16 2n chimeras. Ts 16 2n chimeric mice will provide a valuable tool for studying the functional consequences of aneuploidy and may provide insight into the mechanisms by which trisomy 21 leads to developmental abnormalities in man.     

Partial trisomy 9q: a new syndrome.

Two unrelated patients with a strikingly similar phenotype (low birth weight and poor thriving; mental retardation; dolichocephaly; beaked nose; deeply set eyes; prominent maxilla and receding small chin; long fingers with a peculiar clench) were partially trisomic for two different segments of 9q. The segment found to be trisomic in both patients is small and corresponds to the q31q32 region. This new syndrome is compared to observations of trisomy 9 reported in the literature.     

Meiosis: when even two is a crowd

Recent studies in Caenorhabditis elegans show that crossover interference, which usually limits the number of exchanges per meiotic bivalent to just 'one', requires the continuity of both homologs. One 'function' of crossover interference may be the prevention of crossover events that might not effectively hold homologs together.