Abstract
Speculation about the occurrence of interspecific hybrids in Isoetes began as early as 1896 when Dodge noted that some New England species of Isoetes intergrade at times. Jeffrey and Hicks (1925) and Jeffrey (1937) attributed variation in size and shape of megaspores, lack of protoplasmic content in microspores, and lagging chromosomes during microsporogenesis to hybridization in Isoetes from Nova Scotia. Boom (1980) artificially crossed several Isoetes species. Subsequently, he proposed six interspecific hybrids from southeastern United States (Boom, 1982). However, Kott and Britton (1983) believed that there was insufficient evidence to support interspecific hybridization in northeastern North American Isoetes, and that either ethological or sterility barriers exist among most taxa. Data are presented here to document the natural occurrence of an interspecific hybrid in Isoetes. Hybrids between Isoetes echinospora and I. macrospora were discovered in Neva Lake, Langlade County, Wisconsin. These plants possess megaspores that are more variable in size, shape, and surface ornamentation than typical megaspores of I. echinospora and I. macrospora. The origin of I. echinospora x macrospora is supported by evidence from spore morphology and viability, chromosome counts made from root tip squashes, and electrophoresis of leaf enzymes.
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