Species Integrity and Gene Flow in Anthropomorphic Orchis species

 

I’m working on a group of four orchids called Man Orchid (Orchis anthropophora), Lady Orchid (O. purpurea), Military Orchid (O. militaris), and Monkey Orchid (O. simia), found in England and across mainland Europe. They’re referred to as anthropomorphic because each one has flowers that look like little human figures – they’ve got arms and legs. At first glance, they look easily distinguishable:

 

 

However, when these species grow together, they can’t keep their hands off each other and the different species reproduce with one another. These inter-species partnerships result in intermediate offspring called hybrids that look half like one of the parental orchid species, and half like the other. Take the hybrid Orchis x angusticruris for example, the result of a coupling between a lady orchid and a monkey orchid. The flowers have a distinct halfway feel to them.

 

Orchis x angusticruris (Lady x Monkey Orchid) Hartslock, Oxfordshire 16-05-2009 (2)
Orchis x angusticruris, the hybrid offspring of the Lady Orchid and the Monkey Orchid

 

Where hybridization events such as this occur, it often leads to the build up of large populations of hybrid orchids. Some hybrids appear to be sterile, while others are able to reproduce with each other and with their parents, giving rise to a morphological and genetic spectrum. Very simply, you can imagine it like this: the two parent species are at either end and the hybrids occupy the space in between. Some will be perfect intermediates, some will look more like parent A and some will look more like parent B.

My PhD is basically this: I’m trying to work out why these four orchids remain as separate species, rather than merging into one big hybrid super-species, despite the flow of genes and DNA sequences between them, via the hybrids.

I’m using new DNA sequencing techniques to tease apart relationships among these species and to characterize patterns of hybridization. The things I find out will lead to greater understanding of species coherence in this group, improved understanding of hybridization and better-informed conservation management.

Here are the six possible hybrids: