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Premise: The submersed aquatic plant Hydrilla verticillata ("hydrilla") is important ecologically and economically due to its aggressive growth in both indigenous and nonindigenous regions. Substantial morphological variation has been documented in hydrilla, including the existence of monoecious and dioecious "biotypes." Whereas plastid sequence data have been used previously to explore intraspecific diversity, nuclear data have yet to be analyzed in a phylogenetic context. Molecular and morphological analyses were used to evaluate the genetic diversity and phylogenetic relationships of native and introduced populations., Methods: Nuclear (internal transcribed spacer-ITS; phytoene desaturase-PDS) and plastid (trnL-F) sequence data were evaluated phylogenetically using likelihood and Bayesian methods. Leaf morphologies were compared among clades that were identified in phylogenetic analyses., Results: Data from both ITS and PDS show multiple instances of polymorphic sequences that could be traced to two or more lineages, including both invasive biotypes in the Americas. Leaf morphological data support the distinctness of lineages and provide a metric for distinguishing monoecious and dioecious biotypes in the United States., Conclusions: Nuclear molecular data indicate far greater genetic diversity than could be estimated using plastid markers. Substantially divergent copies of nuclear genes, found in multiple populations worldwide, likely result from interlineage hybridization. Invasive monoecious and dioecious hydrilla biotypes in the Americas are genetically distinct, with both biotypes resulting from admixture among Eurasian progenitors. Genetic similarity to populations in India and South Korea, respectively, implicates these as likely origins for the dioecious and monoecious biotypes., (© 2019 Botanical Society of America.)

Premise of the Study: The discontinuous North American distribution of Najas gracillima has not been explained satisfactorily. Influences of extirpation, nonindigenous introduction, and postglacial migration on its distribution were evaluated using field, fossil, morphological, and molecular data. Najas is a major waterfowl food, and appropriate conservation measures rely on accurate characterization of populations as indigenous or imperiled. •, Methods: Seed lengths of N. gracillima from native Korean populations, a nonindigenous Italian population, and North American populations were compared using digital image analysis. DNA sequence analyses from these regions provided nine nrITS genotypes and eight cpDNA haplotypes. •, Key Results: Najas gracillima seeds from Eurasia and California are shorter than those from eastern North America. Nuclear and chloroplast DNA sequences of N. gracillima from Korea and Italy were identical to California material but differed from native eastern North American plants. Eastern North American specimens of N. gracillima at localities above the last glacial maximum boundary were identical or similar genetically to material from the northeastern United States and Atlantic Coastal Plain and Piedmont but divergent from plants of the Interior Highlands-Mississippi Embayment region. •, Conclusions: In California, N. gracillima is nonindigenous and introduced from Asia. In eastern North America, populations that colonized deglaciated areas were derived primarily from refugia in the Atlantic Coastal Plain and Piedmont. Genetic data indicate initial postglacial migration to northeastern North America, with subsequent westward dispersal into the Upper Great Lakes. These results differentiate potentially invasive California populations from seriously imperiled indigenous eastern North American populations.

The poorly known Haloragaceae R. Br. (Saxifragales) are highly diverse in habit (small trees to submerged aquatics) and labile in floral merosity (2-4), both uncommon among the core eudicots. This family has a cosmopolitan distribution, but taxonomic diversity is concentrated in Australia. An explicit phylogenetic approach has not previously been utilized to examine relationships or character evolution in this family. We used molecular evidence from nrDNA ITS and cpDNA trnK and matK regions under both Bayesian and parsimony analyses to address phylogenetic relationships. Combined molecular analyses defined a monophyletic Haloragaceae with the woody genera (Haloragodendron, Glischrocaryon) sister to the rest. Relationships among many genera were well resolved, with genera as currently delimited generally well supported, although there were notable exceptions; a new genus (Trihaloragis) is recognized, and the aquatic genus Meionectes is again distinct from Haloragis. Three new species combinations are also recognized. There are multiple (two or three) origins of the submerged aquatic habit in the family and potentially an intermediate reversal to the terrestrial habit, neither previously demonstrated in a core eudicot family using an explicit phylogenetic hypothesis. Ancestral character analyses suggest two origins of trimerous flowers and multiple reductions to dimerous flowers throughout Haloragaceae.

Field surveys in eastern North America confirm the naturalization of Glossostigma plants at 19 localities in four states: Connecticut, New Jersey, Pennsylvania, and Rhode Island. DNA sequence analysis of individuals from 14 sampled populations identifies these nonindigenous plants as Glossostigma cleistanthum, a species native to Australia and New Zealand. These results correct prior misidentifications of North American plants as G. diandrum. The earliest North American record of G. cleistanthum (1992) is from a Ramsar tidal wetland in Connecticut. Morphological analyses demonstrate that G. cleistanthum differs from G. diandrum by its longer leaves and ability to produce both cleistogamous and chasmogamous flowers in response to ecological conditions. Glossostigma cleistanthum has a high reproductive potential and spreads rapidly within and between both artificial and natural habitats. A survey of more than 100 lakes indicated that G. cleistanthum occurs most often in waters with high clarity and low pH, alkalinity, conductivity, and phosphorous. Because of its affinity for oligotrophic conditions, this species is a particular threat to pristine natural aquatic communities, which often contain imperiled plants.

The genus Nuphar consists of yellow-flowered waterlilies and is widely distributed in north-temperate bodies of water. Despite regular taxonomic evaluation of these plants, no explicit phylogenetic hypotheses have been proposed for the genus. We investigated phylogenetic relationships in Nuphar using morphology and sequences of the chloroplast gene matK and of the internal transcribed spacer (ITS) regions of nuclear ribosomal DNA. Two major lineages within Nuphar are consistently resolved with the morphological and molecular data sets. One lineage comprises New World taxa and the other represents a primarily Old World lineage. Relationships within the major lineages were poorly resolved by morphology and ITS, yet certain relationships were elucidated by all analyses. Most notable is the strong support for a monophyletic lineage of dwarf taxa and the alliance of the North American N. microphylla with the Eurasian taxa. Minor discordance between the independent cladograms is accounted for by hybridization. The common taxonomic practice of uniting all North American and Eurasian taxa under one species is not supported phylogenetically.

Morphologically heterophyllous species of Potamogeton also commonly display biochemical heterophylly with respect to flavonoid compounds. Generally, floating leaves contain an assortment of flavonoids, whereas submersed leaves often exhibit reduced flavonoid profiles. In strictly submersed (homophyllous) species, two patterns occur. Linear-leaved species have few flavonoids and their biochemical profiles resemble those of submersed leaves of heterophyllous species. Broad-leaved homophyllous species possess flavonoid profiles more similar to those of the floating leaves of heterophyllous species. Numerical analysis of these chemical data is consistent with phylogenetic relationships within the genus derived independently on the basis of morphological and chromosomal data. Glycoflavones, which are probably maintained in floating leaves because of their UV filtering ability, exhibit the most pronounced biochemical heterophylly in Potamogeton. The lack of glycoflavones in submersed leaves of heterophyllous species and in linear-leaved homophyllous species is attributable to the ability of naturally colored water to significantly absorb harmful UV radiation. These observations provide strong support for earlier hypotheses suggesting the importance of flavonoid evolution in the conquest of exposed terrestrial habitats by plants., (© 1990 Botanical Society of America.)