Question

In many parasitic species the sex ratio of males to females is skewed (not 1:1). Parasitic nematodes often have female-biased sex ratios, Can we explain these departure from the Fisherian expectation of 1:1 sex ratio?

a) Individuals within a single host are more closely related - group-level selection (closely related parasites within a host), so having a female biased sex ratio reduces competition among brothers for mates
b) In parasitic nematodes, sex ratios at birth are unbiased and it looks as though observed sex bias is caused by higher female survivorship
c) Female offspring are cheaper to produce than males

Answer 1

The skewed sex ratio in parasitic nematodes is mainly due to group-level selection reducing sibling competition and higher survival rates of females. The cost of producing females over males is not a significant factor for these species.

Step-by-step explanation:

The departure from the Fisherian expectation of a 1:1 sex ratio in parasitic nematodes can be explained by various factors. First, a female-biased sex ratio reduces competition among closely related males within a host (group-level selection). This is beneficial since individuals within a single host are more closely related, and having more females means that the available males have a better chance of mating without intense sibling competition.

Second, in parasitic nematodes, the initial sex ratios at birth might not be biased. However, the observed sex bias could be caused by higher female survivorship. This indicates that while males and females may be born at equal rates, females have a higher chance of surviving to maturity, thus skewing the ratio in favor of females.

Third, there is a possibility that producing female offspring may be less costly than males, but this is not a broadly supported hypothesis in relation to parasitic nematodes. Instead, differences in survivorship and reproductive strategies play more crucial roles in shaping the sex ratio within these species.