Using restriction fragment length polymorphism (RFLP)
we identified 26 unique major histocompatibility complex
(MHC) genotypes in 104 water pythons. We observed a significant
independent association between reduced blood parasite
load (Hepatozoon sp.) and python body length/age, presence
of a specific RFLP fragment (C-fragment) and the overall
number of fragments. The parasite has a negative impact
on several python life-history traits such as growth,
nutritional status and longevity. Thus, the C-fragment
could be considered a 'good gene' (a fitness-enhancing
genetic element). However, while the number of fragments
affected parasite load, the association between level
of parasitaemia and fragment number was not linear, and,
hence, minimum parasite infection level was achieved
at an intermediate number of fragments. Intermediate
MHC fragment numbers were also observed among the largest/oldest
pythons, suggesting that both a specific fragment and
intermediate levels of MHC polymorphism enhanced python
longevity. Thus, our results suggest python MHC is subject
to both frequency-dependent and balancing selection.
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