Abstract
How invasive species overcome challenges associated with low genetic diversity is unclear. Invasive ant populations with low genetic diversity sometimes produce sterile diploid males, which do not contribute to colony labour or reproductive output. We investigated how inbreeding affects colony founding and potential strategies to overcome its effects in the invasive tropical fire ant, Solenopsis geminata. Our genetic analyses of field samples revealed that 13–100% of males per colony (n = 8 males per 10 colonies) were diploid, and that all newly mated queens (n = 40) were single-mated. Our laboratory experiment in which we assigned newly mated queens to nests consisting of 1, 2, 3, or 5 queens (n = 95 ± 9 replicates) revealed that pleometrosis (queens founding their nest together) and diploid male larvae execution can compensate for diploid male load. The proportion of diploid male producing (DMP) colonies was 22.4%, and DMP colonies produced fewer pupae and adult workers than non-DMP colonies. Pleometrosis significantly increased colony size. Queens executed their diploid male larvae in 43.5% of the DMP colonies, and we hypothesize that cannibalism benefits incipient colonies because queens can redirect nutrients to worker brood. Pleometrosis and cannibalism of diploid male larvae represent strategies through which invasive ants can successfully establish despite high inbreeding.
Highlights
Populations arising from an introduction event often lose genetic variation because of their small founder population size[1]
Single-queened S. invicta colonies founded by diploid male producing (DMP) queens were found to have lower growth and survival compared with non-DMP colonies because half of DMP queens’ reproductive output were males that failed to develop beyond the larval stage, instead of workers that could contribute to colony labour[18]
Only DMP S. invicta queens that are adopted into an existing polygyne colony or a queen-less monogyne colony would survive because the worker force supplied by the non-DMP queens would compensate for the production of diploid males from DMP queens[18,21,26]
Summary
Populations arising from an introduction event often lose genetic variation because of their small founder population size[1]. Many populations of successful invaders began as small populations having gone through a genetic bottleneck[4] (e.g. cheatgrass: Bromus tectorum[5], Argentine ant: Linepithema humile[6], house finch: Carpodacus mexicanus[7], solitary sweat bee: Lasioglossum leucozonium[8], Asian honey bee: Apis cerana[9]). In S. invicta, polygyny (multiple-queened colonies) can help minimize the cost of sterile diploid male production induced by low genetic diversity[18]. Single-queened S. invicta colonies founded by diploid male producing (DMP) queens were found to have lower growth and survival compared with non-DMP colonies because half of DMP queens’ reproductive output were males that failed to develop beyond the larval stage, instead of workers that could contribute to colony labour[18]. Queens cooperatively founding a nest (pleometrosis) is another possible means of minimizing the diploid male load during colony founding because DMP queens would benefit from the worker force supplied by non-DMP queens
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