Kirk Roth
Rarely to be taken seriously
With the disclaimer that I haven't read the whole paper yet, so it might have been covered already...
But the lack of F1s suggests to me that they can tell each other apart now, but that they couldn't in the past. This suggests some sort of reinforcement pattern happening in this system, which would be very neat!
Full text of the draft is linked above: https://www.biorxiv.org/content/10.1101/491654v1.full
Its not the final, but I expect few changes.
I wouldn't get your hopes up - I think the mention of no F1s is to suggest that these are populations that have been intergrading for a long time now - ie this is not "new" hybridization nor are hybrids strongly selected against; in either case one would expect to see a lot of first generation hybrids in the population.