When it comes to species coexistence, ecologists and evolutionary biologists both emphasize that there is some interference between taxa and this interference might be more enhanced in closely related species (Gröning and Hochkirch 2008, Pfennig and Pfennig 2009, Grether et al. 2009). Ecological interference may prevent or influence the coexistence of closely related species mainly by resource competition. Reproductive interference in turn is defined as any kind of interaction between species associated with their mating system, which is caused by incomplete species recognition systems and adversely affects the fitness of at least one of the species involved (Gröning and Hochkirch 2008). Because the interference patterns and the resulting selection pressures might be similar, they might be unified as species interference (Pfennig and Pfennig 2009).
Odonata have been used as models in the study of ecology, evolution and behaviour since the 1950’s (Corbet 2004, Córdoba-Aguilar 2008), because of their relatively large body size and their eye-catching reproductive activities. Many mating studies have been based on representatives of the family Calopterygidae, and their biology has been reviewed for Europe (Rüppell et al. 2005) and globally (Córdoba-Aguilar and Cordero Rivera 2005).
The damselfly species Calopteryx splendens and C. virgo have an extensively overlapping distribution. Despite differences in larval ecology (Zahner 1959) and their ecological niches (Wellenreuther et al. 2012), the species frequently inhabit rivers and streams together (Rüppell et al. 2005). The dark-blue wing spots and the elevated courtship behaviour are so obviously sexual selected that they have attracted generations of researchers towards these species (Buchholtz 1951, Heymer 1973, see reviews in Rüppell et al. 2005, Córdoba-Aguilar and Cordero-Rivera 2005). Under moderate male densities, male Calopteryx perform lengthy courtship displays after arrival of females and there is evidence that females chose males on the basis of wing colouration (Siva-Jothy 1999, Svensson et al. 2007). There is strong competition between males over territories, leading to escalated flight-fights between males, including dramatic and energetic costly wing display (for the North American C. maculata see Marden and Waage 1990, Marden 2008). Male disputes over territories are not restricted to intraspecific fights, but males of co-occurring Calopteryx species also compete and fight interspecific over egg-laying spots and sitting positions nearby. Research in Finland has shown, (1) that C. virgo males, which are the larger species, have greater territory-holding ability than sympatric C. splendens males (2) that C. virgo males are most aggressive towards C. splendens with relatively large wing spots (Tynkkynen et al. 2004) and (3) that selection on the size of the wing spots of C. splendens is affected by C. virgo removal (Tynkkynen et al. 2005, 2006). Male wing spot size in C. splendens decreased with increasing relative abundance of C. virgo and these results are viewed as a strong case for reproductive interference, resulting in divergent antagonistic character displacement (Grether et al. 2009).
The authors (Keränen et al. 2012) show that heterospecific sexual interactions occur between two sympatric occurring damselfly species, leading to limited amount of hybridization and introgression. Furthermore using microsatellite markers allowed them to assign the hybrids as F1 or backcrosses. Impressively, the authors were able to sample and analyse more than 2000 individuals, covering males and females of both C. splendens and C. virgo, yielding four hybrids, one of which was identified as a backcrossed female. In contrast to the very low percentage of hybrids in the wild, the authors report a high frequency (six percent) of heterospecific matings during field observations and correlate this number with the number of male densities and OSR-ratios.
As described above, most of the results are robust and are very interesting for the understanding of sympatric Calopteryx interference.
To recall it, the authors report high numbers of interspecific matings, accounting for six percent of all observed matings during their study. Furthermore, all reported heterospecific matings involved C. splendens males and C. virgo females. In contrast, the measured frequency of hybrids detected by microsatellite data was only 0.19 percent. Given the low number of hybrids found, there is discordance between heterospecific mating frequency and the amount of hybridization. One factor discussed in the paper is the 50 percent reduced hatching rate in heterospecific matings between the two Calopteryx species (Lindeboom 1996). If we combine the observed heterospecific mating frequency of six percent with the 50 percent reduced hatching rate (Lindeboom 1996), one can expect approximately three percent hybrids in the population. However, the reported number of hybrids (F1 and backcrosses combined) is only 0.19 %, calling for future studies to explain this gap. As the authors discuss, remating by females before egg laying might be another factor, which might be able to explain the lower than expected amount of hybrids. However, such behavior was not observed in the populations in Finland.
How can we interpret the reported number of six percent heterospecific matings in comparison to other published data? In earlier studies, interspecific matings between the species were not found, and the species isolation was assumed to be complete (Buchholtz 1951). Recent results from the same species pair from Sweden showed, that heterospecific copulations occur in the wild (Svensson et al. 2007). Even if the observed heterospecific mating numbers differed substantially from random mating expectations, these species are not completely isolated from each other. In southern Germany Lindeboom (1996) found heterospecific matings in less than one percent. However the mating asymmetry was opposite to the one in Finland, as all cross-species matings were forced by C. virgo males. There seems to be geographic variation in heterospecific mating frequency and asymmetry. As Referee 2 in PoS outlined, “...the lack of experimental data (in the actual study by Keränen et al. 2012) means that at this point we still can’t be sure that there isn’t some other unmeasured factor which is simultaneously influencing C. virgo density and heterospecific matings. ...., that’s the drawback of a correlational study; it must generally be followed up by experimental data to confirm the mechanism.”
On a minor topic, the authors claim in their abstract: “Studies on interspecific interactions in sympatric species can give information about the maintenance of reproductive isolation, and thus speciation”. Sympatric pairs of Calopteryx species are prime candidates for studying reproductive interference; however, I am not convinced their interference is linked to speciation, as C. splendens and C. virgo have been estimated to be separated since five million years and not considered sister species (Dumont et al. 2005). See Mullen and Andres (2007) for a discussion of a similar situation in two sympatric North American Calopteryx species.
Future studies should include direct manipulation experiments to entangle the mechanisms behind heterospecific matings in the wild. Even so, it would be a good starting point, to study the occurrence of heterospecific matings in relation to the mating strategies of the males (see Tynkkynen et al. 2008). In the actual study the authors present no data, whether heterospecific matings resulted from courtship display or are forced matings by sneakers or were post-egg-laying matings after females returned from underwater or above-water oviposition. The authors claim practical problems against manipulation studies “… it is very difficult to keep the individuals in the lab … An experiment with manipulated densities and relative abundances would be extremely interesting ... It would require a large insectary ...”. I fully agree, that Calopteryx adults are difficult to maintain in the laboratory. However, based on my own experience, adult damselflies thrive well in field based enclosures which must not to be very large or expensive (Lehmann 1995). Therefore, direct manipulation studies might be feasible and I definitely can encourage the authors to do so, as Lindeboom (1996) and Hilfert-Rüppell (2004) effectively used enclosures for Calopteryx.
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