August 11, 2009

Cicadas Are Singing A Different Song

Cicadas - better known for providing the soundtrack of our hot summer are remarkably interesting animals, they are the longest living insects "“ 17 years for some species "“ but spent 99% of this time underground to then emerge for a few weeks, reproduce and finally die. Now a study of north-African and Mediterranean cicadas by scientists in Portugal and the UK uncover yet more interesting data on the group by revealing that these species although differentiated by their mating calls (and genetically) are, nevertheless, morphologically indistinguishable. The work, that resulted in three papers 1-3 published this year, with the last one just out in the European Journal of Entomology3, reveals a remarkable evolutionary process where acoustic characteristics evolve much quicker than any other feature, and are apparently enough to lead to the emergence of new species. The results allow a better understanding of the evolutionary relationships between the species in the group and raise interesting new questions, like how and why this occurred. The work also highlights the importance of mating calls to create genetic isolation and lead to the appearance of new species.

Cicadas are easily recognized by their big wide eyes, medium to large size and acoustic "talent" with groups of singing males able to reach volumes as high as 100 decibels. Despite this visibility they live most of their lives (several years) underground as nymphs, feeding on roots, just to emerge during a hot summer month for a few weeks with a single objective "“ reproduction "“ and then die.  During the little time they spend above ground there is a frantic (and loud) effort for the insects to mate, and for the females to deposit their eggs. The males perform mating songs with the help of a membrane in their abdomen that works like a drum-like organ trying to attract the females of the species, which will only respond to the males of its species.

Cicadas are also particularly good models to study geographic and genetic models of evolution because the different species are easy to distinguish through their songs, mostly sedentary and are sensitive to climate changes (they only reproduce in high temperatures) so in general changes in the environment should be easily traced. An example are the cicadas of the Mediterranean area that, according to their present species distribution, seem to have been influenced by the climate changes of the Pleistocene although the exact details on how this occurred are not clear.

The three papers just published by Gabriela A. Pinto-Juma (1), Sofia G. Seabra (2) and Paula Cristina Simões (3) working under the supervision of Jos© A. Quartau (Centre for Environmental Biology, Lisbon Faculty of Sciences, Portugal) and Michael W. Bruford (School of Biosciences, Cardiff University, Wales, UK) try exactly to understand better the Mediterranean cicadas' relationship, as well as their common evolutionary history and eventually understand better also the evolutionary process. In fact, while Mediterranean cicadas have been divided into species according to their mating calls there is only scarce genetic and no morphological data supporting the division.

So for the last decade Quartau and colleagues have been recording and collecting data on these cicadas and to their surprise these revealed that while the different species - classified according to their mating calls - were in fact different genetically,  morphologically (so in terms of shape and structure) they showed no real differences.

Also interesting were the genetic analyses performed on the animals' mitochondrial DNA. Mitochondria are specialized isolated structures with their own DNA (the only type outside of the nucleus) which is passed from mother to children and can provide "“ by measuring its accumulated mutations "“ an idea on the species age. In fact, the studies revealed that species with continuous songs were older than those with discontinuous calls suggesting that they all have evolved from ancestral(s) with uninterrupted acoustics signals.

In conclusion, Quartau and Bruford's team show that Mediterranean cicadas are characterized by rapid evolution of acoustic signals (apparently from more primitive continuous songs to modern discontinuous calls), which is not accompanied by changes in morphology. This reveals a curious evolutionary path at different velocities and shows how evolution of mating songs are enough to lead to reproductive isolation and genetic differentiation with the appearance of new species.

The results are particularly interesting if we think that acoustic signals require a lot of energy and, still, they appear in several groups of insects "“ cicadas, crickets, etc - indicating they must give some important advantage. The same might apply to changes in the type of songs described in the work now published raising all sort of new questions about how and why this type of evolution occurred.

And as Alberto Quartau says  "contrary to most studies we have acquired in the last 10 years an exhaustive compilation of material, not only by recording cicadas calls but also information on their ecology while collecting the animals for morphological and molecular analyses. This has provided us with a database of linked biologic data that can be accessed to answer new questions and further understand these processes of separation and divergence of the species".

Cicadas have fascinating life cycles and, apparently, also evolutionary history as these new results reveal. The study of different species and their evolutionary history is the basis to understand better evolution itself and the work by Quartau and colleagues is yet another piece in this fascinating puzzle.


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