26 Pages

M Model Organisms

There are tw o sexes - self-fertilizing herm aphrodites and males (Figure 1). M ost adult worms are herm aphrodites, w ith males occurring spontaneously at a rate of <0.5% . However, the progeny of a mating between males and her­ m aphrodites are 50% of each sex. The herm aphrodite lays em bryonated eggs which hatch to give the first larval stage (LI), which then moults through three further larval stages (L2-L4), with the L4 finally m oulting into adult worms (Figure 2). The generation time is about 3 days at 25°C and adult herm aphrodites live for about 10-15 days. There is also an alternative L3 m orph called the dauer larva, which is morphologically, behaviourally and physiologically dis­ tinct from the “norm al” L3. It develops under conditions of crowding and limited food supply and, biologically, it is the resistant, dispersive stage of the life cycle.