Why stream mayflies can reproduce without males but remain bisexual: a case of lost genetic variation

Ephemerella dorothea mayfly dun (image: David Funk)

Continuing our day of fascinating new scientific research on the mayfly, Dr David Funk, insect biologist at the Stroud Water Centre, USA and nature photographer reports on his recent paper “Why stream mayflies can reproduce without males but remain bisexual: a case of lost genetic variation” from the Journal of the North American Benthological Society, explaining the idea of ‘parthenogenesis’…

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Mayflies spend months to years in the water as aquatic larvae, but they are best known for their highly synchronous and often spectacular adult emergences and their extremely short adult lives. For example, individuals of the North American species Dolania americana spend 2 years in the water (8 months as eggs followed by 28 months as larvae) before emerging about an hour before sunrise on one of only about 4 days spread over a two week period in May.  In a desperate rush against time hoards of adults mate and lay eggs. Thousands of spent individuals lie dead on the water surface by the time the sun comes up.

Not all mayfly species are quite this extreme, but the adult life of a mayfly is always brief. Adult mayflies emerge from the water with their eggs ready to be fertilized and laid and, because adults cannot feed (their mouth parts are atrophied), they have a finite amount of energy available with which to accomplish this. Mating typically occurs at highly specific locations and times of day, so females face a very narrow window of opportunity to find a mate.

Ameletus sp. (Image: David Funk)

Inevitably, some females either fail to mate, or their clutches are incompletely fertilized (female mayflies do not have sperm storage organs like most other insects, so sperm usage may be inefficient). For a strictly sexual animal, an unfertilized egg spells the end of the line. But a recent study by Funk et al (2010) showed that a large proportion of mayfly eggs have the ability to self-fertilize in the absence of sperm, and that this mechanism proceeds only after the normal window for fertilization passes.

The development of unfertilized eggs is called parthenogenesis, and the ability for eggs to develop either sexually or parthenogenetically is referred to as facultative parthenogenesis.

The vast majority of mayfly eggs that develop parthenogenetically become females and these individuals are then able to mate and rejoin the sexual population. Thus, facultative parthenogenesis effectively gives unfertilized eggs a second chance.

Theoretically, parthenogenetic reproduction seems to offer an advantage: by not “wasting” reproductive effort on males a parthenogenetic population could potentially increase at twice the rate of a sexual one.  However, Funk and colleagues found that there is a price to pay for parthenogenetic reproduction in these mayflies.

It turns out the parthenogenetic mechanism they employ results in a loss of genetic diversity, anywhere from 10 to 24% per generation. Such a rate of loss probably limits the viability of a strictly parthenogenetic line to only a few generations. Thus facultative parthenogenesis in mayflies appears to be temporary solution for unfertilized eggs.

Most parthenogenetic mayfly eggs develop into females, but a few become intersexes (individuals with both male and female characteristics) and even smaller proportion actually become fully functional males. This raises the possibility that an unmated female mayfly could conceivably establish a new, sexual population on her own.