Analysis of herbarium collections of sporulating Zygodon stirtonii suggests recognition at variety level
Arno van der PluijmA. van der Pluijm (email@example.com), Visserskade 10, NL-4273 GL, the Netherlands.
During revision of Zygodon stirtonii from the Netherlands no intermediates between Z. stirtonii and Z. viridissimus s.s. were found, even when the taxa were growing in mixed populations. From a total number of about 100 Dutch collections of Z. stirtonii, 26 had sporophytes, the majority of them dating back to the 19th century. Sporulating herbarium specimens were analysed for taxonomic and sexual composition. All male, female and sporulating individuals were counted in selected coherent samples from luxurious collections. In small collections, or collections with mostly separate shoots, mostly the presence of taxa was determined. Surprisingly, collections with sporulating plants of Z. stirtonii often also contained Z. viridissimus s.s. with sporophytes. Also, in collections with sporulating plants of one taxon of Zygodon, sometimes only male plants of the other taxon were found intermixed. In collections with plants of both taxa with sporophytes, often male plants of only one taxon were present. It seems likely that Z. stirtonii and Z. viridissimus can cross-fertilize and produce hybrid sporophytes, with viable spores that grow into new generations of both taxa. It is proposed to recognize Z. stirtonii at the variety level.
Up to date maps for Z. viridissimus var. stirtonii and var. viridissimus in the Netherlands are presented. Also, maps of sporulating populations are provided. Zygodon viridissimus var. viridissimus and Z. conoideus have spread rapidly in the Netherlands in recent decades, but var. stirtonii has not. The lack of sporophytes for more than a century may have hampered the spread of var. stirtonii.