Data from: Parasite load and MHC diversity in undisturbed and agriculturally modified habitats of the ornate dragon lizard

MHC gene polymorphism is thought to be driven by host-parasite
coevolution, but the evidence for an association between the selective
pressure from parasites and the number of MHC alleles segregating in a
population is scarce and inconsistent. Here, we characterized MHC class I
polymorphism in a lizard whose habitat preferences (rock outcrops) lead to
the formation of well-defined and stable populations. We investigated the
association between the load of ticks, which were used as a proxy for the
load of pathogens they transmit, and MHC class I polymorphism across
populations in two types of habitat: undisturbed reserves and agricultural
land. We hypothesized that the association would be positive across
undisturbed reserve populations, but across fragmented agricultural land
populations the relationship would be distorted by the loss of MHC
variation due to drift. After controlling for habitat, MHC diversity was
not associated with tick number, and the habitats did not differ in this
respect. Neither did we detect a difference between habitats in the
relationship between MHC and neutral diversity, which was positive across
all populations. However, there was extensive variation in the number of
MHC alleles per individual, and we found that tick number was positively
associated with the average number of alleles carried by lizards across
reserve populations, but not across populations from disturbed
agricultural land. Our results thus indicate that local differences in
selection from parasites may contribute to MHC copy number variation
within species, but habitat degradation can distort this relationship.

主要组织相容性复合体（Major Histocompatibility Complex，MHC）基因多态性被认为由宿主-寄生虫协同进化所驱动，但关于寄生虫施加的选择压力与种群中分离的MHC等位基因数量之间存在关联的证据，不仅稀缺且结论不一。本研究以栖息地偏好为岩石露头、因而形成界限清晰且稳定种群的蜥蜴为对象，对其MHC I类基因多态性进行了表征分析。我们以蜱虫载量作为其传播病原体载量的替代指标，探究了两种生境（未受干扰的保护区与农田）中不同种群的蜱虫载量与MHC I类基因多态性之间的关联。我们提出如下假说：在未受干扰的保护区种群中，二者的关联呈正向；但在碎片化的农田种群中，由于遗传漂变导致的MHC变异丢失，这种关联会被扭曲。在控制生境类型的影响后，MHC多样性与蜱虫数量并无关联，且两种生境在这一方面并无差异。我们也未发现两种生境在MHC多样性与中性遗传多样性的关联上存在差异，该关联在所有种群中均呈正向。不过，个体间的MHC等位基因数量存在广泛差异；我们发现，在保护区种群中，蜱虫数量与蜥蜴携带的平均等位基因数量呈正向关联，但在受干扰的农田种群中则无此关联。综上，本研究结果表明，寄生虫施加的选择压力的局部差异可能促成了物种内的MHC拷贝数变异，但生境退化会扭曲这一关联。