However, recent evolutionary theory predicts that this may be an oversimplification and that less severe stress induces parasites to reduce investment, as a strategy of ‘reproductive restraint’.
This has been interpreted as a strategy of ‘terminal investment’ during extreme stress : investing heavily in gametocytes maximizes transmission potential in a situation likely to be lethal (e.g., before the infection is cleared or the host dies). For example, parasites increase gametocyte investment in response to anaemia, reticulocytes and exposure to sub-lethal anti-malarial therapy. Įvolutionary theory offers explanations for why parasites adjust their reproductive strategies in response to the changing environmental conditions encountered in the host. falciparum sex ratios and gametocyte investment differ between single and mixed infections and are altered in response to variation in RBC density. Observational data from natural infections also suggests that P. Experiments using rodent malaria parasites in vivo and Plasmodium falciparum in vitro suggest that parasites alter investment in gametocytes and their sex ratio in response to: changes in red blood cell (RBC) resource availability, host derived transmission blocking immune (TBI) responses, competition from con-specific genotypes in mixed infections and, drug treatment. Understanding variation in gametocyte investment and sex ratio (collectively referred to as ‘reproductive strategies’) is important because they are key fitness-determining traits, shaping survival within hosts and the success of transmission to new hosts.
has been observed across different species, strains, and during infections. Įxtensive variation in gametocyte investment (also known as the ‘conversion rate’ or ‘reproductive effort’) and sex allocation (proportion of male gametocytes) of Plasmodium spp. These decisions are sensitive to variation in the in-host environment. Specifically, every cell cycle parasites make decisions about how much to invest in gametocytes (which are essential for reproduction and transmission) versus asexuals (which are essential for in-host survival) and in males versus females. This means that, like all sexually reproducing organisms, malaria parasites face resource allocation trade-offs between survival and reproduction and between producing males and females. Discrimination training, in combination with appropriate information regarding writing skills, may help students understand the comments they receive in feedback on written work and thus improve their performance across a range of assessment tasks.Malaria parasites proliferate in the blood through cycles of asexual replication, but every cell cycle a small proportion of progeny commit to developing into male and female gametocytes (which do not replicate in the host). A combination of sex, vocabulary, numeracy and post-test discrimination scores accounted for 43% of the variance in examination performance. Post-test discrimination scores were significant predictors of future written performance, examination performance, and whether students would drop out or fail the examination. There was some evidence that the order in which the two teaching experiences occurred influenced performance. The students’ surface writing skills improved across time, but deep skills did not. A factor analysis of the SSQS revealed two factors, which could be categorised as “surface” and “deep” writing skills. Six weeks after the initial exercise this procedure was repeated, but with the alternative tutorial exercise to that which had been experienced in Week 1. Following this exercise, students completed a brief piece of written work, which was assessed and also analysed using the Six-subscales Quality Scale (SSQS) ( Phadtare et al., 2009).
The students then completed a tutorial exercise in which they either received a traditional presentation regarding writing skills, or were provided with a number of further pairs of examples and asked to discriminate between them with feedback being provided. A brief vocabulary and numeracy test was also conducted. The ability of students enrolled in an introductory psychology unit at the University of Tasmania (N=156) to discriminate good from poor writing was assessed by giving them a number of pairs of expressions that differed in quality and asking them to identify the better example. For students to be able to benefit from feedback provided on their writing, it is necessary for them to be able to discriminate between examples of writing that differ in their quality.
0 kommentar(er)
