Discordance between genetics and morphology suggests hybridization.
Introgressive hybridization seems to be an integral part of evolution. Surprisingly, it has not been reported in the Diplopoda, or millipedes. A recent study in the journal ZooKeys suggests hybridization between two Australian millipedes in the genera Pogonosternum and Somethus. The first report of hybridization in millipedes! Well, there have been other reports, but these turned out to be incorrect.
Peter Decker (Senckenberg Museum of Natural History) collected specimens of P. nigrovirgatum and P. jeekeli in southern Australia, close to the presumed boundary between these species. Next, he sequenced several genetic markers and compared the morphology of these species. The results showed discordance between the genetic and morphological analyses. Could this be due to hybridization?
The discordance was caused by specimens from the site Dargo. Based on the genetic markers, some individuals clustered with P. jeekeli, while others were grouped with P. nigrovirgatum. Morphologically, the Dargo specimens are closest to the latter species. Specifically, their gonopods are very similar to those of P. nigrovirgatum. Gonopods are specialized appendages of various arthropods used in reproduction or egg-laying. Moreover, in millipedes from Dargo you can see brushes on the legs until pair 9, just like in P. nigrovirgatum. In the other species, these brushes only occur until pair 7. It seems likely that these patterns are the outcome of hybridization.
Next, the author compared species of the genus Somethus. In one location – Mount Osmond – he found three individuals of S. castaneus that form their own genetic cluster. These anomalous millipedes might be a distinct lineage of this species or they might have exchanged DNA with another species. More data are necessary to discriminate between these hypotheses, but I wouldn’t be surprised if there was some hybridization involved.
A Personal Note
Although this blog focuses on avian hybrids, I regularly write about hybridization in other taxa. Why did I choose to dedicate a blog post to millipedes? First, because introgressive hybridization has not been reported in this group of animals. When someone finds clues of hybrid millipedes, it is worthwhile to write about it. Second, the paper of this blog post cited my 2016 review on hybridization in geese. It is always nice to see your work cited by other scientists, especially when you least expect it. As a thank you to Peter Decker, I decided to wrote a blog post about his work. So, if you want your work featured on Avian Hybrids, just cite me. 😉
Decker, P. (2018) Phylogenetic and morphological discord indicates introgressive hybridisation in two genera of Australian millipedes (Diplopoda, Polydesmida, Paradoxosomatidae). ZooKeys 809:1-14.