In some taxa of Hymenoptera, fungi, red algae and mistletoe, parasites and their hosts are either sibling species or at least closely related (Emery's rule). Three evolutionary mechanisms have been proposed for this phenomenon: (i) intraspeci c parasitism is followed by sympatric speciation; (ii) allopatric speciation is followed by secondary sympatry and the subsequent parasitism of one sibling species by the other; and (iii) allopatric speciation of a species with intraspeci c parasitism is followed by secondary sympatry, in which one species becomes an obligate parasite of the other. Mechanisms (i) and (ii) are problematic, while mechanism (iii) has not, to our knowledge, been analysed quantitatively. In this paper, we develop a model for single-and two-species evolutionary stable strategies (ESSs) to examine the basis for Emery's rule and to determine whether mechanism (iii) is consistent with ESS reasoning. In secondary sympatry after allopatric speciation, the system's evolution depends on the relative abundances of the two sibling species and on the proportional damage wrought by parasites of each species on non-parasitic members of the other. Depending on these interspeci c effects, either the rarer or the commoner species may become the parasite and the levels of within-species parasitism need not determine which evolves to obligate parasitism.

Exoneura richardsoni is a common allodapine bee which, in southern montane Victoria, primarily nests in dead fronds of tree ferns. We sampled intact colonies from the Dandenong Ranges from August 1992 through to February 1993. The bee is univoltine, with egg production commencing in later winter and adult eclosion occurring over summer. The majority of colonies in both re-used and newly founded nests contain more than one female. Intra-colony relatedness is very high (r = + s.e., 0.759 k 0.087) in re-used nests and moderately high (0.498 i 0.152) in newly cofounded colonies, indicating maintenance of kin integrity during nest re-use and kin recognition during foundress association. This is only the second time that kin cofounding has been demonstrated in a primitively social bee species. Samples during winter and early summer show that per capifa brood production increases with colony size up to about 3 or 4 females per nest, and this may explain the high degree of social nesting in this species. Whilst eusociality could occur in some nests, the opportunities for older, adult brood members to help rear younger siblings are very limited. It seems likely that in E. richardsoni and at least some other Australian allodapine bees, the selective maintenance of high levels of social nesting and kin recognition does not depend on the existence of eusociality.

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Results indicate that prolonged caring for others living with MND has substantial costs for the carer in terms of loss of social support, which affects carer well-being and impacts ultimately on those living with MND. The CNS offers promise as a measure for screening at-risk carers; those who are distressed become candidates for professional intervention to help them cope better. Further research, providing validation of the scale for this task, is recommended.

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