Recombinant yeast as a functional tool for understanding bitterness and cucurbitacin biosynthesis in watermelon (Citrullus spp.).

Cucurbitacins are a group of bitter-tasting oxygenated tetracyclic triterpenes that are produced in the family Cucurbitaceae and other plant families. The natural roles of cucurbitacins in plants are probably related to defence against pathogens and pests. Cucurbitadienol, a triterpene synthesized from oxidosqualene, is the first committed precursor to cucurbitacins produced by a specialized oxidosqualene cyclase termed cucurbitadienol synthase. We explored cucurbitacin accumulation in watermelon in relation to bitterness. Our findings show that cucurbitacins are accumulated in bitter-tasting watermelon, Citrullus lanatus var. citroides, as well as in their wild ancestor, C. colocynthis, but not in non-bitter commercial cultivars of sweet watermelon (C. lanatus var. lanatus). Molecular analysis of genes expressed in the roots of several watermelon accessions led to the isolation of three sequences (CcCDS1, CcCDS2 and ClCDS1), all displaying high similarity to the pumpkin CpCPQ, encoding a protein previously shown to possess cucurbitadienol synthase activity. We utilized the Saccharomyces cerevisiae strain BY4743, heterozygous for lanosterol synthase, to probe for possible encoded cucurbitadienol synthase activity of the expressed watermelon sequences. Functional expression of the two sequences isolated from C. colocynthis (CcCDS1 and CcCDS2) in yeast revealed that only CcCDS2 possessed cucurbitadienol synthase activity, while CcCDS1 did not display cucurbitadienol synthase activity in recombinant yeast. ClCDS1 isolated from C. lanatus var. lanatus is almost identical to CcCDS1. Our results imply that CcCDS2 plays a role in imparting bitterness to watermelon. Yeast has been an excellent diagnostic tool to determine the first committed step of cucurbitacin biosynthesis in watermelon.

Egg-Laying Behavior of Cataglyphis niger Ants Is Influenced More Strongly by Temperature Than Daylength.

Temperature and photoperiod are the two most important factors that affect all aspects of animal life. We conducted two experiments to examine the effect of temperature and photoperiod on egg laying and development in the desert ant Cataglyphis niger. In the first experiment, we examined the effect of decreasing temperatures and shortening daylength on egg-laying behavior. An additional treatment was exposure to natural autumn conditions. Decreasing temperatures impaired egg laying much more than shortening daylength. The effect, however, was rapidly reversible when raising the temperature. When the outdoor treatment was brought inside the lab at a suitable temperature, queens started laying eggs as well. In the second experiment, we first kept the colonies under warmer temperatures and moved them gradually to cooler temperatures, 1-20 days after the eggs were laid. The probability of eggs developing into larvae and pupae under cooler temperatures was positively influenced by the exposure duration to warmer temperatures before the temperature switch. When the eggs developed into larvae, longer exposure to warmer temperatures before the temperature switch led to faster development. However, when the eggs disappeared (and were probably eaten), longer exposure to warmer temperatures before the temperature switch led to slower egg disappearance. We suggest that the decision to lay eggs is reversible to some extent because the workers can consume the eggs if conditions deteriorate. We suggest that this reversibility reduces the cost of laying eggs at the wrong time.