Below are brief descriptions of published results from my PhD work and other research related to Plasmodium mexicanum that I have been involved in. The central question in my PhD research was:
Does the Local Mate Competition of ratio theory explain the
variation in sex ratios observed in malaria parasites?
See my section on sex ratio theory if you're interested in learning more about the model predictions.
Single-Clone Infections (PDF)
Malaria parasites reproduce asexually, so sometimes all parasites in an infection arose from one cell cloned many times. Local Mate Competition (LMC) predicts female-biased sex ratios in single-clone infections.
Sex ratios were female-biased, as predicted, and varied among clones. Sex ratios did not appear to change predictably over time. The image shows sex ratios of individual infections (points) of different clones (x axis) with median and 95% interval (diamonds).
Male Fecundity (PDF)
LMC predicts that, in single-clone infections, just enough males will be produced to mate with all females. Because observed sex ratios are only slightly female-biased (~40-45% male), we expect male gamete production (fecundity) to be low.
Male fecundity was low- each gametocyte produces only 2-3 gametes on average- and appeared to vary among infections (see image- points are average gametes/gametocyte, lines are 95% confidence interval; left group are 1 clone infections, right group are multi-clone). Sex ratios of single-clone infections predicted by male fecundity match those observed well if only ~40% of gametes are successful.
Observed sex ratios may be unpredictable by LMC if they are developmentally linked to other traits and therefore shaped by unforeseen selective forces. We measured 10 life history traits (e.g. infection growth rate, time to produce gametocytes) and used principal components analysis to group them.
No life history traits grouped with sex ratio, suggesting that it is an independent trait. The variation in sex ratios among clones of parasite mentioned above (Single-Clone Infections) was observed again. The figure shows variation in principal components (named by the life history traits that explain the most variation) among clones/genotypes. Significant variation among genotypes occurred only for sex ratio.
Clonal Diversity and Sex Ratio (manuscript PDF)
LMC predicts higher (less female-biased) sex ratios in infections with higher clonal diversity (i.e. infections with many equally abundant clones). We tested this with three data sets: natural infections sampled over 3 years and experimental infections from 2 previous studies. Only those infections with the highest and lowest third of clonal diversity measures were compared.
No difference between high and low diversity infections was detected for natural infections (panel b in the figure), but there was a significant difference in the predicted direction for both sets of experimental infections (panels d and f).