Sex Allocation in Hermaphrodites

Abstract

Organisms in which individuals can reproduce both as males and females are called hermaphrodites. Sex allocation in hermaphrodites involves the division of reproductive resources between the male and female function, and presents an interesting contrast to species that alter sex allocation by adjusting offspring sex ratio. Theoretical and empirical research in the past four decades have largely attempted to explain when hermaphroditism is favoured, and how sex allocation in hermaphrodites is controlled. Furthermore, the application of endocrinological methods has elucidated some of the molecular processes that underlie hermaphroditic sex allocation, especially in sequential hermaphrodites. Despite significant advances in our understanding of hermaphrodites, some fundamental predictions such as the adaptiveness of changes in sex allocation have not been tested, and some basic questions on hermaphroditism have not been answered. Key concepts: In hermaphroditic organisms, individuals are capable of reproducing as both males and females. Simultaneous hermaphrodites have male and female reproductive organs that are functional at the same time, and are both involved in a mating event. Simultaneous hermaphroditism is favoured, when an increase in male and female reproduction yields diminishing fitness returns. Factors such as limited mobility, low density or self‐fertilization can result in diminishing fitness returns for males, and favour simultaneous hermaphroditism. However, local resource competition and physiological limitations could saturate fitness through female function. Simultaneous hermaphrodites can adjust sex allocation according to the social environment (e.g. mating group size), and the amount of resources available for reproduction (e.g. body size). Sequential hermaphrodites, or sex changers, reproduce as one sex for a part of their lifetime, and then switch to reproducing as the opposite sex. Sequential hermaphroditism is favoured in mating systems where individuals have consistently greater reproductive success as one sex earlier and as the other sex later in life. Sequential hermaphrodites can adjust the optimal timing of sex change according to relative size or social status in a mating group. There are also mixed hermaphroditic systems where individuals can repeatedly switch between sexes, or simultaneous hermaphrodites coexist with pure sexes. Sex change in fishes involves complex behavioural and morphological changes that are regulated by an endocrine gland system conserved across all vertebrates.

Sex allocation and investment into pre- and post-copulatory traits in simultaneous hermaphrodites: the role of polyandry and local sperm competition

“…Although work on sex allocation has predominantly focused on gonochorists, simultaneous hermaphrodites are also well suited to test sex allocation theory, and recent reviews show that sex allocation research on simultaneous hermaphrodites is making substantial progress, both in plants [10] and animals [11] (for recent reviews on sequential hermaphrodites, which we do not cover here, see [12,13]; for simplicity, we hereafter refer to simultaneous hermaphrodites as hermaphrodites). We primarily focus on copulating simultaneous hermaphrodites, by which we mean animals where the sperm donor engages in some form of direct sperm transfer to a sperm recipient (including itself in the case of selfing).…”