Isoetes Xbrittonii hyb. nov. (Isoetaceae): A Naturally Occurring Hybrid (I. engelmannii X I. riparia) in the Eastern United States

Abstract

Although Jeffrey (1937) provided a convincing cytological and morphological argument for the existence of hybrids between natural populations of Isoetes in North America over 50 years ago, a systematic and broadly-based investigation of this phenomena has been undertaken only recently. Boom (1980; 1982) showed that various eastern North American species could be hybridized in culture. On the basis of herbarium studies he also reported several naturally occurring hybrid combinations. Taylor et al. (1985) established geographical, cytological and electrophoretic evidence for the existence of naturally occurring Iso6tes hybrids. They showed that the uncommon northeastern North American diploid taxon I. x eatonii Dodge is a sterile hybrid between I. echinospora Dur. and I. engelmannii A. Br., and they made in vitro hybridizations between these parental species. Two other naturally occurring Iso6tes hybrids from northeastern North America have been recently discovered. Taylor and Luebke (1988) reported I. x hickeyi (I. echinospora x macrospora [lacustris]) from Wisconsin and Britton & Brunton (1989) identified I. x dodgei A. A. Eaton (pro sp.) as I. echinospora x riparia Dur. from Ontario, Quebec, Vermont and New Hampshire. All of these interspecific hybrids were initially detected by their production of irregular megaspores which are more varied in size, shape and surface ornamentation than megaspores of parental species. This paper describes another Isoetes hybrid from northeastern North America. it is believed to be the result of hybridization between I. engelmannii and I. riparia. Nine plants of this hybrid have been found, each demonstrating combinations of the characters expressed by Isoetes hybrids, viz, plants with hybrid vigour, aborted spores of variable size and shape, and spores with surface ornamentation patterns more or less intermediate between the putative parents (Jeffrey, 1937; Taylor & Luebke, 1988; Britton & Brunton, 1989). Further evidence supporting the hybrid nature of this taxon comes from its chromosome number, its sporadic occurrence within the partially sympatric ranges of its putative parents and its constant association with one of its parents at each station.