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The genetics of thelytoky… when sex doesn’t pay
Sex is the predominant form of reproduction among multicellular organisms, yet the near ubiquity of sexual reproduction remains an enduring evolutionary mystery. When a female reproduces sexually she halves her genetic contribution to her offspring by combining her genome with that of her male partner who generally contributes nothing in return. This ‘two-fold cost’ is at the heart of the 'Paradox of Sex'. While a number of hypothetical benefits of sex have been proposed, it is not currently clear whether any are sufficiently powerful to explain its prevalence. Sex plays a pivotal role in shaping the natural world, yet we remain unable to fully comprehend how sex evolved or why it is maintained.
During my honours project I investigated the endurance of an asexual lineage of the Cape honey bee, Apis mellifera capensis. The Cape honey bee is unique among honey bees because workers are capable of thelytoky, i.e. they effectively clone themselves, laying unfertilised eggs that develop into female offspring. Cape workers utilise this ability to compete with their queen and fellow workers over the maternity of new queen and entirely thelytokous parasitic lineages regularly emerge. One such lineage currently exists as a reproductive parasite of their neighbouring subspecies A. m. scutellata, aka the African Killer bee. This 'Clone' lineage has endured without sex for 20 years and is a substantial economic burden on the honey producing industry of South Africa. I identified a substantial cost to thelytoky in the 'Clone' lineage: effective inbreeding that results in decreased brood viability. I propose that this cost is endured by virtue of a parasitic life-history, which is characterised by high rates of reproduction, low maternal investment and concordantly high early mortality. Simply put, when the vast majority of offspring will never survive to maturity, it doesn’t matter if a proportion are inviable.
During my PhD project I am continuing this work in the Cape honey bee. I am extending my investigation into the genetics of thelytoky and the evolutionary tradeoffs between sex and asexuality within the 'Clone' lineage and also within the 'normal' Cape population. I utilise genetic, cytological, modelling and behavioural techniques to better understand the consequences of thelytoky in the Cape honey bee. I also hope to extend my line of enquiry to a broader, cross-taxa approach, characterising the evolutionary tradeoffs between sex and asexuality and the genetic consequences of thelytoky. A particular area of interest will be that of the thelytokous ants. Known thelytokous ant species now number at least 10, having almost doubled in the past 10 years. This trend looks set to continue and is rapidly revealing an astoundingly diverse array of reproductive strategies, many of which would not have been conceived of only a few years ago.
Thelytoky represents the exception that potentially reveals the rule. Understanding what sets thelytokous lineages apart from the sexual majority allows us to unravel the 'Paradox of Sex'. Eusocial taxa such as A. m. capensis are ideally suited to this pursuit and possess hitherto largely unrealised potential to illuminate the evolutionary tradeoffs that underpin the distribution of reproductive strategies across taxa.