Equality between the sexes in plants for costs of reproduction; evidence from the dioecious Cape genusLeucadendron(Proteaceae)

Abstract

10It has been argued that sexual allocation is greater for female function than male function in 11 plants in general and specifically for the large dioecious Cape genus Leucadendron. Here, 12 we use new interpretations of published information to support the hypothesis of equality 13 between sexes in this genus. The explanations are based on the fire ecology of the Cape that 14 results in reproductive synchrony, reproductive doubling and competitive symmetry. Firstly, 15 strict post-fire seedling establishment of the reseeder life-history in the Cape results in single-16 aged populations. Consequently, the reproductive and vegetative schedules of males must 17 synchronously track that of females. This implies equal allocation to sex. Secondly, after 18 fires, dioecious females have double the seedling to adult ratio of co-occurring 19hermaphrodites. This indicates that being liberated from male function allows females access 20 to resources that double their fitness compared to hermaphrodites. Therefore, male and 21 female costs of reproduction are equal in hermaphrodites. Thirdly, competitive symmetry 22 must occur because males and female plants will frequently encounter each other as close 23 near neighbours. Competitive asymmetry would both reduce mating opportunities and skew 24 local sex ratios. The evidence to date is for 1:1 sex ratios in small plots and this indicates 25 competitive symmetry and a lack of dimorphic niches. Finally, vegetatively dimorphic 26 species must also allocate equally to sex, or else sexual asynchrony, lack of reproductive 27 doubling or competitive asymmetry will occur. 28 29 30 31Understanding allocation by the different sexes is a key focus in evolutionary studies and in 32 organisms where both parents contribute genes equally to off-spring, sexual allocation (e.g. 33 sex ratios, parental care of off-spring of different sexes, costs of reproduction) is expected to 34 be equal (Fischerian) (Hamilton 1967; Charnov 1982). It is difficult to investigate sexual 35 allocation in plants because most plants (>90%) are hermaphrodites, nevertheless, it is 36 generally assumed that the resource costs of reproduction are greater for females (Barrett and 37 Hough 2013). Although only female plants bear the costs of producing large seeds, fruits and 38 cones after flowering, it is nevertheless controversial whether total female costs are higher 39 than male costs. For example, Harris and Pannell (2010) argued that serotiny (storage of 40 seeds in closed cones that open after plant death in fire) is a form of maternal care that 41 imposes extra selection pressure on females of the Cape Proteaceae genus Leucadendron for 42 greater water efficiency. They also argue that vegetative dimorphism, such as the larger 43 leaves and lower rates of branching in females, results from this extra ecophysiological 44 selection on females. Cones on females need to maintain a water supply to prevent them from 45 drying out during the dry, hot Mediterranean summers and opening prematurely and thus 46 relea...