Interplay of two distant loci
Essentially, self-incompatibility is based on the interplay of the two loci – the S locus and the Z locus – which are located on different chromosomes.
The genes are the blueprint for three different proteins, which form a kind of lock-and-key mechanism that recognises whether the pollen that has landed on the stigma is genetically similar or unrelated. This sets off a signal that either aborts the fertilisation process or continues it to completion.
Studer and his team are currently studying the structures of these proteins and how they interact to differentiate between foreign pollen and the plant’s own pollen. For this, they use special artificial intelligence methods to model the structure of the corresponding proteins based on the gene sequence, along with models that predict the interactions between these molecules.
A unique self-incompatibility mechanism
In addition, the researchers have studied how self-incompatibility based on two loci could have evolved in the grass family, as all other mechanism known from other plant families are based on one locus only. It’s probable that in the evolutionary history of grasses, the Z locus initially duplicated, and that the copy then underwent numerous mutations, leading to diversification.
'We’ve now sequenced the two loci in a great many grass plants. What we’ve found is that the S locus tends to have lower sequence variation and is still diversifying, while the Z locus doesn’t change as much. From this we conclude that the Z locus might be older in evolutionary terms,' Studer explains.
By tracing the phylogeny of grasses, the researchers have also learned when the locus duplication occurred and when the species diverged from one another. Moreover, the phylogenetic tree revealed which grasses did not undergo locus duplication and which species have lost their self-incompatibility, for instance through mutation.
But what is the evolutionary benefit of self-incompatibility based on two loci? 'At first glance, we assume that it opened up a lot more possibilities and flexibility for plants in the grass family to recognise their own pollen,' Studer says. This could have been important for the grass family, whose 16,000 species are distributed across all continents, making it one of the world’s largest and most successful plant families.