The Hybrid Polypody: Polypodium X incognitum


I have been wondering for years why our hybrid Polypodium is so abundant, especially in southeastern New England.

Hybrid ferns are usually quite rare. In Dryopteris they do occur, and two of them - D. X boottii and D. X triploidea - are more common than the rest. One can usually find a few where the parents both grow nearby. But the hybrid Polypody is the predominate taxon in some locations, without the obvious presence of both or even one of its parents. Clearly it has some means of reproduction beyond the chance mating of the two parent species.

I have seen amber colored sperical "spores" among the whitish shriveled heterogeneous spores of the hybrid. I imagine they are unreduced mother cells capable of growing apogamous offspring. Sometimes I notice several on a single frond. Still, they would need to have quite a high success rate to account for the very large communities the hybrid sometimes produces.

I don't really know why P. appalachianum is more abundant in the northern three New England states, and P. virginianum in the southeast. Apparently there is some selective advantage, possibly involving minimum winter temperature. It is plausible that the hybrid retains whatever disadvantage P. virginianum has, especially as they have the same genetic components, though in different quantity.

P. virginianum is an allotetraploid, the product of a chromosome-doubling mutation long ago in a hybrid between P. appalachianum and P. sibericum, now a species of the Pacific Northwest. The genetic makeup of these taxa can be depicted as follows:



P. sibericum P. appalachianum P. virginianum P. X incognitum


For the last several years I have been attempting to gather the under-reported taxa, P. appalachianum and P. X incognitum, from as many New England counties as I can. I describe the project here.