B chromosomes as a hidden driver of intestinal nematode infection in the yellow-necked mouse (Apodemus flavicollis).

The host-parasite relationship is affected by various biological, ecological, and anthropogenic factors. Some of them, such as host sex, are well studied. On the other hand, the effect of genetic differences, such as the presence of supernumerary or B chromosomes (Bs), has received little attention. We investigated the independent and interactive effects of host sex and B chromosome presence on intestinal nematode abundance in 294 yellow-necked mice (Apodemus flavicollis) from 18 localities in Serbia. Nine nematode species were identified. Because parasite numbers showed high inter-individual variability, we used a Monte Carlo randomisation method to test differences in mean abundance without assuming a specific statistical distribution. Host sex had a limited influence on infection patterns. In line with existing theoretical frameworks, a significant male-biased abundance was detected, but only for Aonchotheca annulosa, whereas no overall sex bias was found for total nematode burden. B chromosome carriers were found to have a significantly higher abundance of Syphacia frederici. A significant interaction between sex and Bs was detected for total intestinal nematodes and Syphacia stroma, with females carrying Bs harbouring the highest parasite loads. The results, indicating that Bs can shape helminth infrapopulation structure and that their effects may be sex-dependent, were discussed. The absence of consistent male-biased parasitism further suggests that sex effects are parasite-specific rather than universal. Our study highlights the importance of considering cryptic genomic variation in host-parasite research and demonstrates the value of Monte Carlo methods for analysing overdispersed helminth abundance data in wildlife hosts.