In Drosophila melanogaster, the maternally inherited endocellular microbe Wolbachia causes cytoplasmic incompatibility (CI) in crosses between infected males and uninfected females. CI results in a reduction in the number of eggs that hatch. The level of CI expression in this species has been reported as varying from partial (a few eggs fail to hatch) to nonexistent (all eggs hatch). We show that male age in this host species has a large impact on the level of CI exhibited and explains much of this variability. Strong CI is apparent when young males are used in crosses. CI declines rapidly with male age, particularly when males are repeatedly mated. Wolbachia from a Canton S line that was previously reported as not causing CI does in fact induce CI when young males are used in crosses, albeit at a weaker level than in other D. melanogaster strains. The strain differences in CI expression are due to host background effects rather than differences in Wolbachia strains. These results highlight the importance of undertaking crosses with a range of male ages and nuclear backgrounds before ascribing particular host phenotypes to Wolbachia strains.

DNA variation was studied in a 2.2 kb region of the regulatory gene Atmyb2 using 20 ecotypes of Arabidopsis thaliana and one accession each of Arabis gemmifera and Arabidopsis himalaica. Nucleotide diversity (π) in the region was 0.0027, which was lower than for other loci in A. thaliana. The MYB domain of the Atmyb2 gene (π = 0.0036) had a larger variation than the non-MYB region (π = 0.0013). Tajima's test and Fu and Li's test did not give a significant result. In contrast to the low level of polymorphism, the degree of divergence of the Atmyb2 region was higher between A. thaliana and A. gemmifera (K = 0.0730) than for other loci. The MYB domain (K = 0.0436) had smaller divergence than the non-MYB region (K = 0.0939). The HKA test detected significant discordance in the ratio of polymorphism to divergence in some comparisons. The pattern of low polymorphism and high divergence, which is mainly observed in the non-MYB region of the gene, is inconsistent with the neutral mutation theory. Strong purifying selection after establishment of A. thaliana and a species-specific adaptive process could be invoked to account for this pattern of polymorphism and divergence of Atmyb2.

In the first 25 generations of his classical mutation accumulation experiment, T. Mukai estimated a large rate of early linear decay for the relative viability of Drosophila melanogaster chromosome II (ΔMII = 0.004). Mukai forced through zero the regression of viability decline on generation number, but it has recently been shown (Fry, 2001) that a similar decline (ΔMII = 0.006) is obtained from unforced regression even if generation 32 instead of generation 25 (whose validity has been questioned) is included. We show that, from the perspective of the whole long-term experiment, it is hard to decide up to which generation viability can be considered to decline linearly. Depending on this decision, and on whether or not the regression is forced through the origin, very different estimates are obtained. Furthermore, the particular behaviour of the lines used as control suggests that they could have been different from the remaining lines at the beginning of the experiment, and casts doubts on the adequacy of a forced regression. Estimates from the linear unforced regression (ΔMII = 0.011) or from the linear term in a quadratic unforced regression (ΔMII = 0.001) are very different. The data fit both models very well, and the choice between them should be based on biological grounds.

We investigate the joint effects of gene flow and selfing on the level of inbreeding depression, heterosis and genetic load in a subdivided population at equilibrium. Low gene flow reduces inbreeding depression and substantially increases heterosis. However, in highly self-fertilizing populations, inbreeding depression is independent of the amount of gene flow. When migration occurs via pollen, consanguinity of the reproductive system could have a negative influence on subpopulation persistence, in contrast to the case of isolated populations. However, with only seed migration, genetic load and heterosis depend mildly on the mating system. From an evolutionary point of view, we reach two main conclusions: first, outcrossing is selected for if gene flow is low; second, intermediate levels of gene flow could promote mixed mating systems, especially when migration occurs through pollen.

An unusual chromosomal hybrid zone of the house mouse, Mus musculus domesticus, exists in Upper Valtellina, Northern Italy, consisting of four Robertsonian (Rb) races and the standard (all-acrocentric, or 2n = 40) race, all hybridizing freely within 10 km2. The hybrid zone in Valtellina provides an excellent opportunity to study the role of Rb fusions in reproductive isolation and speciation. This hybrid zone has already been well studied for the distribution of Rb fusions and the fertility of hybrids, but in order to understand the dynamics of the zone, a basic understanding of the origin and genetic similarity of the chromosomal races is necessary. This paper presents the results of three different methods of measuring genetic differentiation: multivariate analysis of morphological traits and analyses of allozyme variation and mitochondrial DNA sequences. The standard race is clearly distinguishable from the three Rb races by all three methods, but the Rb races are not distinguishable from one another. This provides strong evidence for our previous suggestions that the well-established Rb races in Valtellina are closely related, and that the standard race was introduced into the valley more recently from a distant source. The fact that the Rb races are indistinguishable is also consistent with our hypothesis that a within-village speciation event involving two of the races (Hauffe & Searle, 1992) was a recent occurrence. The low level of allozyme heterozygosity among the Rb races suggests that these populations are the products of at least one bottleneck. The present article substantially extends earlier studies and provides the first detailed morphological and molecular analysis of this complex hybrid zone.

Two molecular maps of Triticum monococcum L were produced and integrated. The integrated map includes a total of 477 markers, 32 RFLPs, 438 AFLPs, one morphological (soft glume (Sog)) and six storage-protein markers, and covers 856 cM. The trait Sog with the recessive allele sog maps to linkage group 2S. Probably, this is the T. monococcum homologue of Tg and Tg2 in hexaploid and tetraploid wheats, respectively. Loci coding for seed storage proteins were allocated to chromosomes 1L (HMW GLU1,2 and Glu1), 1S (LMW GLU6,7, LMW GLU1-4, ωGLI1-4, γGLI5 and Gli-1) and 6L (α/βGLI7-14). Parameters related to bread-making quality (SDS sedimentation volume, specific sedimentation volume (SSV) and total protein content) were studied in one of the two populations. A QTL that is consistently present across environments was detected for SDS sedimentation volume and for SSV. The position of the QTL on chromosome 1S was in close agreement with the map positions of storage-protein loci. A second QTL was mapped on chromosome 5. For protein content, two significant QTLs were mapped to linkage groups 1 and 5.

Results from a QTL experiment on growth and carcass traits in an experimental F2 cross between Iberian and Landrace pigs are reported. Phenotypic data for growth, length of carcass and muscle mass, fat deposition and carcass composition traits from 321 individuals corresponding to 58 families were recorded. Animals were genotyped for 92 markers covering the 18 porcine autosomes (SSC). The results from the genomic scan show genomewide significant QTL in SSC2 (longissimus muscle area and backfat thickness), SSC4 (length of carcass, backfat thickness, loin, shoulder and belly bacon weights) and SSC6 (longissimus muscle area, backfat thickness, loin, shoulder and belly bacon weights). Suggestive QTL were also found on SSC1, SSC5, SSC7, SSC8, SSC9, SSC13, SCC14, SSC16 and SSC17. A bidimensional genomic scan every 10 cM was performed to detect interaction between QTL. The joint action of two suggestive QTL in SSC2 and SSC17 led to a genome-wide significant effect in live weight. The results of the bidimensional genomic scan showed that the genetic architecture was mainly additive or the experimental set-up did not have enough power to detect epistatic interactions.