AboutZoos, Since 2008


Redefin­ing ‘Species’: a new and con­tro­ver­sial concept

pub­lished 12 March 2017 | mod­i­fied 12 March 2017

What is a species? Biol­o­gists — and ornithol­o­gists in par­tic­u­lar — have been debat­ing the best def­i­n­i­tion for a very long time. A new com­men­tary pub­lished on 8 March in The Auk: Ornitho­log­i­cal Advances pro­poses a novel con­cept: that species can be defined based on the unique co-​adaptations between their two genomes, one in the nuclei of their cells and the other in their mitochondria.

All ani­mals have two sets of genes, one in the cell nucleus and one in organelles called mito­chon­dria, and these two sets of DNA work together to enable cel­lu­lar res­pi­ra­tion and energy pro­duc­tion. If they’re mis­matched, the result is reduced energy out­put and increased pro­duc­tion of dam­ag­ing free rad­i­cals. While the most com­monly used species def­i­n­i­tion is based on the idea that iso­lated pop­u­la­tions slowly accu­mu­late changes in their nuclear genes that make inter­breed­ing impos­si­ble, Auburn University’s Geof­frey Hill pro­poses a new twist on the species con­cept — that spe­ci­a­tion is really the diver­gence of sets of co-​adapted mito­chon­dr­ial and nuclear genes. Inter­species hybrids, his the­ory sug­gests, have reduced fit­ness due their mis­matched genomes’ reduced abil­ity to work together in the cell.

mitochondrial dnaAn overview of the genomic archi­tec­ture of birds (top) and the func­tional are­nas in which mitonu­clear inter­ac­tions are man­i­fest (bot­tom). Nuclear genes and their prod­ucts are shaded blue; mito­chon­dr­ial genes and their prod­ucts are shaded green. Sources: num­ber of genes from Pra­chumwat and Li (2008); num­ber of N-​mt genes and N-​mt protein-​coding genes from Bar-​Yaacov et al. (2012); num­ber of NO-​mt genes from Bur­ton and Bar­reto (2012).
Source: Geof­frey E. Hill Depart­ment of Bio­log­i­cal Sci­ences, Auburn Uni­ver­sity, Auburn, Alabama, USA (2017) The mitonu­clear com­pat­i­bil­ity species con­cept. The Auk: April 2017, Vol. 134, No. 2, pp. 393409.

Past stud­ies have shown that mito­chon­dr­ial geno­type tends to be very good at show­ing species bound­aries between birds. This “mitonu­clear com­pat­i­bil­ity species con­cept” helps explain the fact that the abrupt tran­si­tions between mito­chon­dr­ial geno­types at species bound­aries cor­re­spond with abrupt tran­si­tions in songs, plumage pat­terns, and female mat­ing pref­er­ences. Inter­est­ingly, two closely related species that have recently been doc­u­mented to have exten­sively inter­min­gled nuclear genes — Blue-​winged and Golden-​winged war­blers — also show an abrupt tran­si­tion in mito­chon­dr­ial genes.

Almost all ornithol­o­gists who write and think about avian spe­ci­a­tion study phy­lo­geog­ra­phy — the geo­graph­i­cal dis­tri­b­u­tion and genetic struc­ture of bird pop­u­la­tions,” says Hill. “In con­trast, I study bird orna­men­ta­tion and, par­tic­u­larly, bird coloura­tion. It was the dis­cov­ery that orna­ments sig­nal mito­chon­dr­ial type that led to my sud­den real­iza­tion that mito­chon­dr­ial type or, more accu­rately, co-​adapted sets of mito­chon­dr­ial and nuclear genes, define species bound­aries. I don’t think I would have ever seen the pat­tern if I had come at the ques­tion from a phy­lo­geo­graphic perspective.”

This is an intrigu­ing and con­tro­ver­sial idea — that mitonu­clear incom­pat­i­bil­i­ties could be so cen­tral to gen­er­at­ing new avian species — and I see this as a call for more research into how these incom­pat­i­bil­i­ties might man­i­fest them­selves in young species,” says avian evo­lu­tion­ary biol­o­gist David Toews of Cor­nell Uni­ver­sity. “The func­tional aspects of mito­chon­dr­ial genes have, in par­tic­u­lar, received lit­tle atten­tion from the ornitho­log­i­cal com­mu­nity, and it will be inter­est­ing to see how these ideas play with addi­tional empir­i­cal stud­ies going forward.”

(Source: The Auk Ornitho­log­i­cal Advances press release, 06.03.2017)

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