Distribution and taxonomy of Isoetes tuckermanii subsp. acadiensis, comb. nov. (Isoetaceae) in North America

Résumé

Isoetes acadiensis is an emergent aquatic lycophyte of freshwater shores found in a narrow range along the Atlantic coast of northeastern North America where it frequently coexists with Isoetes tuckermanii (sensu stricto [s. str.]). Apparently fertile plants with intermediate morphology occur commonly in mixed populations. No sterile hybrids between the two taxa have been detected. Although I. acadiensis maintains a distinctive geographic distribution (within and smaller than that of I. tuckermanii [s. str.]), exhibits molecular evidence of genetic distinctiveness, and has morphologically distinctive features in most populations, the weight of evidence suggests it is not distinct from I. tuckermanii at a species level. Accordingly, I. tuckermanii subsp. acadiensis, comb. nov. is proposed as the appropriate designation for this biogeographically important Acadian endemic.