Monday, 20 April 2015

Males Are Here To Stay: Sex Enhances Egg Production And Colony Fitness

To us humans, it seems extremely unnatural that other animals can reproduce without having sex. Yet with the passing of time, evolution has endowed females of several species of amphibians, insects, reptiles and fish the ability to asexually produce offsprings without "help" from males.

Now, researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) say that in such animals, fertilization still has an important role, ensuring the survival of the maximum number of healthy offsprings. Their research appears online in The Science of Nature.

Generally, most species capable of parthenogenesis (asexual reproduction) use it to increase their numbers faster in harsh environments when females can't find males. Asexual reproduction also comes handy when many females but few males enter a new habitat.

Scientists speculate that this ability originally arose in many species, either due to conflict between the sexes or to ensure survival when mates were scarce. Some of these species now consist entirely of females.

The OIST Ecology and Evolution Unit has looked at the early evolutionary transition from sexual reproduction to clonal reproduction by studying a special case: the Little Fire Ant. Also known as the electric ant (Wasmannia auropunctata), it is a small (approx 1.5 mm long), light to golden brown social ant native to Central and South America, now spread to parts of Africa (including Gabon and Cameroon), North America, Puerto Rico, Israel, and six Pacific Island groups plus north-eastern Australia.

Little fire ant bities a human
Little fire ant biting a human
By Plegadis (Own work) [Public domain]

W.auropunctata is a species in which certain populations reproduce sexually and others clonally, yet colonies of both types still have males.

As is the case of other ants, the males of this species fertilize queens to produce a worker class where all individuals are sterile, and accept genetic contributions from both parents. But here is where the twist comes.

In W.auropunctata fertile males hatch with no genetic contribution from the queen laying the egg, and new queens hatch without genetic contribution from the father. But why do males still exist, and mating still takes place in a species where the male and female genetic information stays separate and queens also have the ability to clonally produce both queens and workers?

To answer this, the OIST researchers studied colonies from both sexually and clonally reproducing populations. They discovered that inseminated queens had almost a 100 percent success rate in terms of how many of their eggs hatched, whereas in the case of queens that remained virgin, from both clonally and sexually reproducing populations, the majority of the eggs did not make it past early stages of embryo development. Furethermore, queens that haid sex laid eggs faster than the virgin ones.

 little fire ant worker foraging for food on a leaf's surface
Little fire ant worker foraging for food

The study strongly suggests that in the Little Fire Ant case the battle of sexes is unlikely. Mating with males enables the queen to produce more healthy offsprings than by cloning itself. Sex also increases a queen's fitness as indicated by the enhanced egg laying and hatching success rates and as a result, the colony's fitness.

"In case of the whiptail lizard in the New Mexican desert which consists only of females, a type of pseudo-copulation still takes place before eggs start developing, suggesting evolution places certain checks on completely eliminating sex and sexual behavior," said Prof. Alexander Mikheyev, head of OIST's Ecology and Evolution Unit, the paper's co-author.

The mechanisms by which evolution prevents sex from being completely eliminated still remain largely mysterious. But what is certain is that even though some species can bypass the need for sex to deal with challenging conditions, an evolutionary constraint seems to be wiring the females in these species to still expect sexual stimulus. Without it, their reproductive systems just don't perform as well.

- Miyakawa MO, & Mikheyev AS (2015). Males are here to stay: fertilization enhances viable egg production by clonal queens of the little fire ant (Wasmannia auropunctata). Die Naturwissenschaften, 102 (3-4) PMID: 25801787

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