Evolutionary biology

Island of the clones

The discovery of an all-female population of damselflies in the Azores archipelago provides a novelty for entomologists. It also highlights the unique selection pressures faced by species that colonize islands.

Tucked away in the journal Odonatologica comes a paper by Cordero Rivera and colleagues1 that will surprise many entomologists, and will exercise biologists studying evolution on islands and the mechanisms of sex determination. Cordero Rivera et al. have discovered that a species of damselfly on the Azores reproduces parthenogenetically (Fig. 1). This form of reproduction, in which females produce eggs that develop without fertilization by males2, has been recorded in almost all insect groups. But until now it was not known to occur in any natural populations of damselflies or dragonflies (the Odonata)3.

Figure 1: Reproduction without fertilization in a damselfly.
figure1

A female Ischnura hastata lays eggs in a pond on the island of Pico, Azores. (Courtesy of A. Cordero Rivera, Univ. Vigo.)

The Azores archipelago lies 1,500 km from the coast of Europe. Inspired by a report4 that only females of the damselfly Ischnura hastata had ever been found there, Cordero Rivera and his team visited 15 localities on six of the islands. Although more than 330 adult specimens of I. hastata were examined, none of them was male. To test whether the species was parthenogenetic, a sample of larvae was reared to adulthood in the laboratory — more than 1,900 females were produced over nine generations, but no males.

Ischnura hastata is common in North and South America, yet it occurs in these regions as a classically sexual species with both males and females. The concept of ‘geographic parthenogenesis’5 proposes that the parthenogenetic forms of a species are more likely to occur in certain areas — such as higher latitudes and altitudes, and on islands — because of the different selection pressures that organisms face under these conditions6,7. One possibility, therefore, is that certain damselfly species can include both sexual and parthenogenetic forms, and that on arriving on a remote island it is the parthenogenetic form that is favoured, at least initially, owing to the difficulty of finding mates.

One might wonder why standard sexual reproduction does not kick in once the population builds up in size, but perhaps local conditions continue to favour parthenogenesis. Indeed, I. hastata frequents temporary or recently established habitats4, and Cordero Rivera et al. note that there is anecdotal evidence of local extinctions of pond populations. Furthermore, chance may play a role in the establishment and maintenance of parthenogenesis: I. hastata is also found on the Galapagos Islands, but the population contains both males and females8.

In at least some odonates, there may be a degree of predisposition to parthenogenesis; for example, there is evidence that unfertilized eggs of the dragonfly Stylurus oculatus can be artificially induced to develop9. Moreover, certain parasites that are inherited only in the female line can manipulate their insect host into producing predominantly (or only) female offspring10. Cordero Rivera and colleagues are testing whether any microbial agents are responsible for driving the absence of males in I. hastata, but they have ruled out one potential bacterial parasite, Wolbachia, which infects a range of other insect groups10. If parthenogenesis in I. hastata is parasite mediated, then the microbial agent might have had a beneficial effect on its host in the initial phases of colonization, allowing individuals to reproduce without mates.

There have also been intriguing accounts of other damselfly species on remote archipelagos. In particular, on the islands of Fiji, it seems that females of the damselfly Nesobasis rufostigma actively defend territories over aquatic habitats, whereas the males, which are infrequently encountered, reside some distance from the stream11. This phenomenon has been dubbed ‘sex-role reversal’11 and, if confirmed, would be the first example in an odonate. If males are in short supply, then this unusual mating system might be explained by female competition for access to males12. Furthermore, males of two rarer Fijian damselflies (N. flavostigma and N. caerulescens) have never been found, raising the possibility that parthenogenesis occurs in these species.

Clearly, the evolutionary reasons why parthenogenesis is maintained in I. hastata remain largely unsolved. Entomologists, and those interested in island biology and parasite-mediated sex determination, have a new case for investigation.

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Sherratt, T., Beatty, C. Island of the clones. Nature 435, 1039–1040 (2005). https://doi.org/10.1038/4351039a

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