The genus Diphasiastrum belongs to one of the oldest lineages of extant terrestrial plants, the class Lycopodiophyta (also known as Lycophyta)[1]. In Systematics, Diphasiastrum has been recognized as a section of the genus Lycopodium L. (Lycopodium sec. Complanata), but due to certain features (branchlet structure, chromosome number [n = 23], and carrot-shaped gametophytes), they are now treated as a genus Diphasiastrum[2]. With 27 described taxa[3], including hybrids, the most complex group within the Lycopodiaceae, Diphasiastrum is distributed widely throughout the Northern Hemisphere.
Unlike many other plants that commonly undergo allopolyploid hybridization rusulting in numerous polyploid individuals, Diphasiastrum exhibits only few polyploid individuals[4, 5, 6] and homoploid hybrid speciation (diploid homoploid species) is supposedly predominant, even though for some hybrid taxa seemed to be originating from polypoid hybridization (e.g. D. x wightianum, D. x zanclophyllum, and D. x yueshanense)[4, 7]
In homoploid hybridization, backcrossing occurs more easily than allopolyploid hybridization, resulting in the frequent introgression[6, 8] and, therefore, contributing to species diversification. This relatively unique evolutionary mechanism among plants provides a valuable model for studying homoploid hybrid speciation and its role in adaptive radiation.
[1] Wikström, N., & Kenrick, P. (2001). Evolution of Lycopodiaceae (Lycopsida): Estimating divergence times from rbcL gene sequences by zse of nonparametric rate smoothing. Molecular Phylogenetics and Evolution, 19(2), 177–186. https://doi.org/10.1006/mpev.2001.0936
[2] Holub, J. (1975). Diphasiastrum, a new genus in Lycopodiaceae. Prestia, 47, 97–110.
[3] Horn, K. (2021). Die Flachbärlappe (Diphasiastrum spp., Lycopodiaceae, Lycopodiophyta) Mitteleuropas. Taxonomie, Biologie, Verbreitung und Gefährdung. University of Greifswald.
[4] Aagaard, Sunniva. M. D., Greilhuber, J., Zhang, X. C., & Wikström, N. (2009). Occurrence and evolutionary origins of polyploids in the clubmoss genus Diphasiastrum (Lycopodiaceae). Molecular Phylogenetics and Evolution, 52(3), Article 3. https://doi.org/10.1016/j.ympev.2009.05.004
[5] Aagaard, S. M. D., Vogel, J. C., & Wikström, N. (2009). Resolving maternal relationships in the clubmoss genus Diphasiastrum (Lycopodiaceae). TAXON, 58(3), Article 3. https://doi.org/10.1002/tax.583012
[6] Bennert, H. W., Horn, K., Kauth, M., Fuchs, J., Jakobsen, I. S. B., Ollgaard, B., Schnittler, M., Steinberg, M., & Viane, R. (2011). Flow cytometry confirms reticulate evolution and reveals triploidy in Central European Diphasiastrum taxa (Lycopodiaceae, Lycophyta). Annals of Botany, 108(5), 867–876. https://doi.org/10.1093/aob/mcr208
[7] Schnittler, M., Inoue, M., Shchepin, O. N., Fuchs, J., Chang, H., Lamkowski, P., Knapp, R., Horn, K., Bennert, H. W., & Bog, M. (2024). Hybridization and reticulate evolution in Diphasiastrum (flat-branched clubmosses, Lycopodiaceae) – New data from the island of Taiwan and Vietnam. Molecular Phylogenetics and Evolution, 196, 108067. https://doi.org/10.1016/j.ympev.2024.108067
[8] Hanušová, K., Ekrt, L., Vít, P., Kolář, F., & Urfus, T. (2014). Continuous Morphological Variation Correlated with Genome Size Indicates Frequent Introgressive Hybridization among Diphasiastrum Species (Lycopodiaceae) in Central Europe. PLoS ONE, 9(6), e99552. https://doi.org/10.1371/journal.pone.0099552
1. Population genetics and biogeography of Diphasiastrum complanatum in the Northern Hemisphere
2. Hybridization status within genus Diphasiastrum in the Northern Hemisphere
3. Status Diphasiastrum x issleri in Central Europe
4. Speciation status genus Diphasiastrum in Japan