Hybridising mosquitoes provide a new perspective on the biological species concept

A new study pub­lished in Evol­u­tion Let­ters has shown that the form­a­tion of new spe­cies does not neces­sar­ily require com­plete gen­om­ic isol­a­tion. Author Prof. Markus Pfen­ninger tells us more about the findings.

How we think about spe­cies, spe­ci­ation and hybrid­isa­tion is not only of aca­dem­ic interest, but also influ­ences soci­ety and polit­ics. Bio­lo­gic­al jus­ti­fic­a­tions for racism and the rejec­tion of inter­ming­ling are essen­tially based on the seem­ing rar­ity of pair­ings between spe­cies and the allegedly neg­at­ive con­sequences for their offspring.

We now know that hybrid­iz­a­tions are more the rule than the excep­tion in the anim­al king­dom and that spe­ci­ation can also take place with con­tin­ued genet­ic exchange. The pre­vail­ing the­ory sug­ges­ted that reg­u­lar gene flow through suc­cess­ful pair­ings between pop­u­la­tions is the main bar­ri­er to their segreg­a­tion into spe­cies. In recent years, how­ever, the­or­et­ic­al and empir­ic­al work has shown that spe­ci­ation is pos­sible under cer­tain cir­cum­stances des­pite gene flow. It is not clear how wide­spread this phe­nomen­on of spe­ci­ation with gene flow is and how the spe­ci­ation pro­cesses take place over time. In par­tic­u­lar, it is unknown wheth­er diver­gence pro­cesses once begun will always and inev­it­ably lead to com­plete gen­om­ic and repro­duct­ive isol­a­tion soon­er or later. If they do, this would mean that the diver­ging pop­u­la­tions even­tu­ally become “real” spe­cies in the sense of the widely recog­nized bio­lo­gic­al spe­cies concept.

In our recently pub­lished com­par­is­on of the gen­omes of indi­vidu­als of two reg­u­larly hybrid­iz­ing sis­ter spe­cies of mos­qui­toes (Chiro­nomus ripari­us and C. piger), we show that these spe­cies formed with­in a short peri­od, mil­lions of gen­er­a­tions ago. The two spe­cies occur in a spa­tial mix with one anoth­er through­out the north­ern hemi­sphere, but prefer eco­lo­gic­ally dif­fer­ent habitats.

A male Chiro­nomus ripari­us sit­ting on a graph­ic­al rep­res­ent­a­tion of its gen­ome. Photo: Markus Pfenninger.

The spe­ci­ation pro­cess between the two spe­cies appar­ently stopped com­pletely at some point, even before the entire gen­ome was mutu­ally isol­ated. Hybrid­iz­a­tion has occurred reg­u­larly since then, i.e. they can and con­tin­ue to mate. The two spe­cies reg­u­larly exchange a good 70% of their gen­ome, which con­tains around half of all genes. The taxa remain eco­lo­gic­ally recog­nis­able, although they only have 30% of the gen­ome (albeit with the oth­er half of the genes) exclus­ively for them­selves. These isol­ated areas are quite small and scattered across the genome.

Half of the genes that are not exchanged are there­fore appar­ently respons­ible for keep­ing the spe­cies dif­fer­ent. Apart from genes that are related to the known eco­lo­gic­al dif­fer­ences between the spe­cies (e.g. sens­it­iv­ity to nitro­gen com­pounds such as nitrite from manure), these are very often genes that have to work closely with oth­er genes at the molecu­lar level. Such genes can be found, for example, in the res­pir­at­ory chain, pro­tein pro­duc­tion or in gates through mem­branes. At these points, incom­pat­ib­il­it­ies between the dif­fer­ent spe­cies would obvi­ously be too disturbing.

Our res­ults were able to show that the form­a­tion of spe­cies does not neces­sar­ily have to end in their com­plete isol­a­tion. Rather, there are stable inter­me­di­ate stages in which related, well-char­ac­ter­ized spe­cies can gain and main­tain their diversity and still share most of their gen­ome with one anoth­er. This changes our view of what we call bio­lo­gic­al species.

Prof. Markus Pfen­ninger is Head of the Molecu­lar Labor­at­ory in the Depart­ment of Molecu­lar Eco­logy, Senck­en­berg Biod­iversity and Cli­mate Research Centre. The ori­gin­al study is freely avail­able to read and down­load from Evol­u­tion Letters.