A project undertaken at the School of Biological Sciences, The University of Queensland, and supervised by Dr Lyn Cook and Dr Alicia Toon
Bush coconuts (or bloodwood apples) are galls induced by scale insects on bloodwood eucalypts in northern and eastern Australia. The galls can be the size of a cricket ball, and both the internal gall tissue and the female that causes the gall are considered bush tucker (Figure 1). The female scale insect starts to induce the gall when she is a tiny nymph (only 200 microns long) and will spend the rest of her life enclosed within her gall, growing up to 4 cm in length. After mating, she first produces male offspring that feed on the nutritious white tissue that lines the inside of the gall. When the males reach maturity and pupate, the adult female gives birth to her tiny daughters. These climb onto the elongate abdomens of their winged adult brothers and are carried away to a new host plant (Figure 2).
If carrying sisters is costly to a male and decreases his chances of finding a mate, there must be some trade-off between carrying sisters or going it alone in order for the life-history to have evolved and to be maintained. Almost all relatives of bush coconuts have a chromosomal system called paternal genome elimination (PGE), such that males pass on only their mothers’ gene copies (the fathers’ copies are destroyed before making sperm). PGE systems result in unequal contributions of the mother and father to the next generation, and are thought to result from intersexual conflict.
Through this project, we found that two of the three species of Cystococcus exhibit a PGE system. This means that males of these two species only pass on their gene copies through their daughters. We also found that, although females can produce many thousands of offspring, sons are largely the result of a mating with a single male. This means that sons will mostly be transporting sisters that carry gene copies from that son’s own father rather than those on another male. Both these results are consistent with the evolution of behaviours in sons to help their sisters, such that the father’s gene copies are passed on to future generations via his daughters
Figure 1.A pair of bush coconuts from north Queensland.
Figure 2. A bush coconut opened to reveal the wrinkly adult female in the centre. The white tissue inside the gall inspired the common name “bush coconut” or “bloodwood apple”.
Figure 3. Winged males, carrying tiny sisters on their abdomens, escape from a hole cut in a gall by a researcher.
The third species of Cystococcus appears to have “lost” PGE and there must be a novel method of male determination because both sexes are diploid—such an occurrence is known only from two other species of scale insect. During field work we also discovered a new-to-science species of bush coconut from the Northern Territory. Both these new discoveries will need to be further investigated.