Genetic conflict outweighs heterogametic incompatibility in the mouse hybrid zone?

<p>Abstract</p> <p>Background</p> <p>The <it>Mus musculus musculus/M. m. domesticus </it>contact zone in Europe is characterised by sharp frequency discontinuities for sex chromosome markers at the centre of wider clines in allozyme frequencies.</p> <p>Results</p> <p>We identify a triangular area (approximately 330 km²) where the <it>musculus </it>Y chromosome introgresses across this front for up to 22 km into <it>domesticus </it>territory. Introgression of the Y chromosome is accompanied by a perturbation of the census sex ratio: the sex ratio is significantly female biased in <it>musculus </it>localities and <it>domesticus </it>localities lacking Y chromosome introgression. In contrast, where the <it>musculus </it>Y is detected in <it>domesticus </it>localities, the sex ratio is close to parity, and significantly different from both classes of female biased localities. The geographic position of an abrupt cline in an X chromosome marker, and autosomal clines centred on the same position, seem unaffected by the <it>musculus </it>Y introgression.</p> <p>Conclusion</p> <p>We conclude that sex ratio distortion is playing a role in the geographic separation of speciation genes in this section of the mouse hybrid zone. We suggest that clines for genes involved in sex-ratio distortion have escaped from the centre of the mouse hybrid zone, causing a decay in the barrier to gene flow between the two house mouse taxa.</p

Testing parasite 'intimacy': the whipworm Trichuris muris in the European house mouse hybrid zone.

Host-parasite interaction studies across hybrid zones often focus on host genetic variation, treating parasites as homogeneous. 'Intimately' associated hosts and parasites might be expected to show similar patterns of genetic structure. In the literature, factors such as no intermediate host and no free-living stage have been proposed as 'intimacy' factors likely constraining parasites to closely follow the evolutionary history of their hosts. To test whether the whipworm, Trichuris muris, is intimately associated with its house mouse host, we studied its population genetics across the European house mouse hybrid zone (HMHZ) which has a strong central barrier to gene flow between mouse taxa. T. muris has a direct life cycle and nonmobile free stage: if these traits constrain the parasite to an intimate association with its host we expect a geographic break in the parasite genetic structure across the HMHZ. We genotyped 205 worms from 56 localities across the HMHZ and additionally T. muris collected from sympatric woodmice (Apodemus spp.) and allopatric murine species, using mt-COX1, ITS1-5.8S-ITS2 rDNA and 10 microsatellites. We show four haplogroups of mt-COX1 and three clear ITS1-5.8S-ITS2 clades in the HMHZ suggesting a complex demographic/phylogeographic history. Microsatellites show strong structure between groups of localities. However, no marker type shows a break across the HMHZ. Whipworms from Apodemus in the HMHZ cluster, and share mitochondrial haplotypes, with those from house mice. We conclude Trichuris should not be regarded as an 'intimate' parasite of the house mouse: while its life history might suggest intimacy, passage through alternate hosts is sufficiently common to erase signal of genetic structure associated with any particular host taxon

Intensity of infection with intracellular Eimeria spp. and pinworms is reduced in hybrid mice compared to parental subspecies

Genetic diversity in animal immune systems is usually beneficial. In hybrid recombinants, this is less clear, as the immune system could also be impacted by genetic conflicts. In the European house mouse hybrid zone, the long‐standing impression that hybrid mice are more highly parasitized and less fit than parentals persists despite the findings of recent studies. Working across a novel transect, we assessed infections by intracellular protozoans, Eimeria spp., and infections by extracellular macroparasites, pinworms. For Eimeria, we found lower intensities in hybrid hosts than in parental mice but no evidence of lowered probability of infection or increased mortality in the centre of the hybrid zone. This means ecological factors are very unlikely to be responsible for the reduced load of infected hybrids. Focusing on parasite intensity (load in infected hosts), we also corroborated reduced pinworm loads reported for hybrid mice in previous studies. We conclude that intensity of diverse parasites, including the previously unstudied Eimeria, is reduced in hybrid mice compared to parental subspecies. We suggest caution in extrapolating this to differences in hybrid host fitness in the absence of, for example, evidence for a link between parasitemia and health.Peer Reviewe

Spatiotemporal patterns of egg laying in the common cuckoo

Understanding egg-laying behaviour of brood parasites in space and time can improve our knowledge of interactions between hosts and parasites. However, no studies have combined information on the laying activity of an obligate brood parasite with detailed information on the distribution of host nests within an area and time period. Here, we used molecular methods and analysis of egg phenotypes to determine maternal identity of common cuckoo, Cuculus canorus, eggs and chicks found in the nests of four species of Acrocephalus warblers in consecutive years. The median size of a cuckoo female laying area (calculated as a minimum convex polygon) was correlated negatively with the density of host nests and positively with the number of eggs assigned to a particular female. Cuckoo female laying areas overlapped to a large extent and their size and location did not change between years. Cuckoo females preferentially parasitized host nests located close to their previously parasitized nests and were mostly host specific except for two that parasitized two host species. Future studies should focus on sympatric host and parasite communities with variable densities across different brood-parasitic systems to investigate how population density of hosts affects fitness and evolution of brood parasites. For instance, it remains unknown whether female parasites moving to new sites need to meet a threshold density of a potential host. In addition, young females may be more limited in their egg laying, particularly with respect to the activity of other parasites and hosts, than older females

The house mouse chromosomal hybrid zone in Valtellina (SO): a summary of past and present research

&lt;strong&gt;Abstract&lt;/strong&gt; Karyotypic variation due to the centric (Robertsonian: Rb) fusion of chromosomes is a widespread phenomenon among small mammal species. In 1993, we described a house mouse chromosomal hybrid zone in Upper Valtellina (SO). Here, we found mice with 32 different karyotypes, including the standard, or all-acrocentric race (2n=40), four Rb races (2n=22-26) and 27 hybrid types (2n=23-39). This hybrid zone presents a unique opportunity to study the role of Rb fusions and races in speciation. We have been studying this dynamic hybrid system using a wide variety of techniques: karyology, histology, breeding, mark-recapture and DNA sequencing. All four Rb races appear to be closely related, but 40AA has probably been introduced recently into the valley. However, the fertility of laboratory-reared hybrids between several of these races (24UV, 26POS, 40AA) are lower than expected compared to homozygotes and previous studies. Effective subpopulation size and migration rates within and between villages are also relatively low. We discuss the use of these parameters to study the process of speciation in ongoing computer simulations

Kropackova-et-al-data

Data on fetal and placental weights in F1 crosses and backcrosses. X chromosome genotypes of backcross individuals

Data from: Maternal-fetal genomic conflict and speciation: no evidence for hybrid placental dysplasia in crosses between two house mouse subspecies

Interspecific hybridization between closely related mammalian species, including various species of the genus Mus, is commonly associated with abnormal growth of the placenta and hybrid fetuses, a phenomenon known as hybrid placental dysplasia (HPD). The role of HPD in speciation is anticipated but still poorly understood. Here we studied placental and fetal growth in F1 crosses between four inbred mouse strains derived from two house mouse subspecies, Mus musculus musculus and M. m. domesticus. These subspecies are in the early stage of speciation and still hybridize in nature. In accordance with the maternal-fetal genomic conflict hypothesis we found different parental influences on placental and fetal development, with placental weight most affected by the father's body weight, and fetal weight by the mother's body weight. After removing the effects of parents’ body weight, we did not find any significant differences in fetal or placental weights between intra-subspecific and inter-subspecific F1 crosses. Nevertheless, we found that the variability in placental weight in inter-subspecific crosses is linked to the X chromosome, similarly as for HPD in interspecific mouse crosses. Our results suggest that maternal-fetal genomic conflict occurs in the house mouse system, but has not yet diverged sufficiently to cause abnormalities in placental and fetal growth in inter-subspecific crosses. HPD is thus unlikely to contribute to speciation in the house mouse system. However, we cannot rule out that it might have contributed to other speciation events in the genus Mus, where differences in the levels of polyandry exist between the species