Some species of Daphnia become more armored in the presence of predatory fish. I

Question

Some species of Daphnia become more armored in the presence of predatory fish. In one species the ability to produce armor seems to have a genetic basis, in that some families when grown in predator conditioned water (water that had fish in it but now does not) will consistently produce armor (plastic phenotype), while others grown in the same type of water will not (fixed phenotype). You make many crosses between plastic and fixed phenotype Daphnia and find that the offspring are always of the fixed phenotype. When you make many crosses of the offspring to each other you get the following results. 614 fixed, 287 plastic, 53 intermediate (intermediate have slight armor development). What can you conclude about the genetic basis of armor and why?

Explanation / Answer

Non-genetic transgenerational modifications of offspring

phenotype are increasingly evident in physiological studies.

Indeed, this phenomenon is emerging as a potential source of

variation in comparative physiology. Here we focus on

non-genetic transgenerational transfer of morphological,

physiological and behavioral traits in the zebrafish (Danio rerio)

and the water flea (Daphnia magna). The experimental design

was similar in both studies (Ho and Burggren, 2012;

Andrewartha and Burggren, 2012). Essentially, parents were

chronically exposed to hypoxia and then returned to normoxia

for breeding and reproduction. A control population stayed in

normoxia. The subsequently produced offspring (6-18 day old

zebrafish larvae; 0-18 day old Daphnia larvae) were then

exposed to severe hypoxia and their responses recorded.

Additionally, physiological and metabolic traits of the larvae

whose parents were exposed to hypoxia were assessed and

compared with control populations. In Danio, larval offspring

had longer body length when derived from adults that had been

exposed to hypoxia for 2, 3 or 4 weeks. Hypoxic resistance

(measured by time to loss of equilibrium) 6-18 dpf was ~15%

lower in those larvae from parents that had been exposed to 1

week of chronic hypoxia, but longer exposures (2,3 or 4 weeks)

significantly increased larval resistance by ~24-30%. CTMin

(~39.5

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