Donald H. Les

Phylogenetic Studies in Alismatidae, II: Evolution of Marine Angiosperms (Seagrasses) and Hydrophily

Aquatic species represent fewer than two percent of all flowering plants, and only 18 aquatic genera have acquired true hydrophily (water-pollination) which is associated with an unusually high incidence of unisexual flowers. From the subset of submersed, hydrophilous angiosperms, only 13 genera have colonized marine habitats. The evolution of hydrophily, unisexuality, and marine habit in angiosperms was explored using estimates of phylogeny obtained by phylogenetic analyses of chloroplast (rbcL) gene sequence data. Despite what might appear to be difficult evolutionary transitions, hydrophiles are highly polyphyletic with independent origins in the monocotyledon subclass Alismatidae in addition to two derivations in the dicotyledon families Ceratophyllaceae and Callitrichaceae. Yet, even in alismatids, hydroph- ily has evolved many times. Unisexuality has also evolved repeatedly in the Alismatidae, and is ancestral to the evolution of hydrophiles and marine plants in the Hydrocharitaceae. Marine angiosperms (known only from Alismatidae) have evolved in three separate lineages. The multiple origins of hydrophilous, marine plants offer an extraordinary example of convergent evolution in angiosperms.

Molecular phylogenetics of the Magnoliidae: cladistic analyses of nucleotide sequences of the plastid gene rbcL

Nucleotide sequences of the plastid protein-coding gene rbcL from 64 species of 36 families in subclass Magnoliidae sensu Cronquist and representatives of all other major seed plant groups were analyzed by parsimony in a series of four analyses. Ceratophyllum (Ceratophyllaceae) was found to be sister to all other angiosperms. Other magnoliids formed five major groups, roughly corresponding to the Magnoliales, Laurales, Aristolochiaceae/Piperales, Nymphaeales, and Ranunculales/Papaverales. Four magnoliid lineages, those with monosulcate or monosulcate-derived pollen (Magnoliales, Laurales, Aristolochiales/Piperales, and Nymphaeales), and the monocots (with the same type of pollen) formed a weakly supported monophyletic group

The Central Role of Dispersal in the Maintenance and Persistence of Seagrass Populations

Global seagrass losses parallel significant declines observed in corals and mangroves over the past 50 years. These combined declines have resulted in accelerated global losses to ecosystem services in coastal waters. Seagrass meadows can be extensive (hundreds of square kilometers) and long-lived (thousands of years), with the meadows persisting predominantly through vegetative (clonal) growth. They also invest a large amount of energy in sexual reproduction. In this article, we explore the role that sexual reproduction, pollen, and seed dispersal play in maintaining species distributions, genetic diversity, and connectivity among seagrass populations. We also address the relationship between long-distance dispersal, genetic connectivity, and the maintenance of genetic diversity that may enhance resilience to stresses associated with seagrass loss. Our reevaluation of seagrass dispersal and recruitment has altered our perception of the importance of long-distance dispersal and has revealed extensive dispersal at scales much larger than was previously thought possible.

Evolution of Aquatic Angiosperm Reproductive Systems

vironments and became aquatic. Aquatic plants are species that perpetuate their life cycle in still or flowing water, or on inundated or noninundated hydric soils. Aquatic angiosperms inhabit oceans, lakes, rivers, and wetlands. The transition to an aquatic life has been achieved by only 2% of the approximately 350,000 angiosperm species (Cook 1990). Nonetheless, the evolutionary invasion of aquatic environments by terrestrial angiosperms is estimated to represent 50-100 independent events (Cook 1990). Although aquatic plants are typically discussed as a unified biological group, the ways that species have evolved to life in the aquatic milieu are as diverse as the different

Evidence of hybridity in invasive watermilfoil (Myriophyllum) populations

Invasions of nonindigenous species have caused ecological devastation to natural communities worldwide, yet the biological bases for invasiveness remain poorly understood. Our studies of invasive watermilfoil (Myriophyllum) populations revealed widespread polymorphisms in biparentally inherited nuclear ribosomal DNA sequences, which were not detected in populations of native North American species. Subclones of the polymorphic regions revealed the occurrence of distinct sequences matching those acquired from both nonindigenous and native North American species. Molecular data demonstrate clearly that invasive watermilfoil populations in North America have resulted from hybridization between nonindigenous and native species. These observations suggest that invasiveness in these aggressive aquatic weeds may be linked to heterosis maintained by vegetative propagation.

Phylogeny and Systematics of Lemnaceae, the Duckweed Family

Abstract The minute, reduced plants of family Lemnaceae have presented a formidable challenge to systematic investigations. The simplified morphology of duckweeds has made it particularly difficult to reconcile their interspecific relationships. A comprehensive phylogenetic analysis of all currently recognized species of Lemnaceae has been carried out using more than 4,700 characters that include data from morphology and anatomy, flavonoids, allozymes, and DNA sequences from chloroplast genes (rbcL, matK) and introns (trnK, rpl16). All data are reasonably congruent (I(MF) < 6%) and contributed to strong nodal support in combined analyses. Our combined data yield a single, well-resolved, maximum parsimony tree with 30/36 nodes (83%) supported by bootstrap values that exceed 90%. Subfamily Wolffioideae is a monophyletic clade with 100% bootstrap support; however, subfamily Lemnoideae represents a paraphyletic grade comprising Landoltia, Lemna, and Spirodela. Combined data analysis confirms the monophyly of Landoltia, Lemna, Spirodela, Wolffia, and Wolffiella. Phylogenetic relationships are used to evaluate and refine the classification of duckweeds. Communicating Editor: Jeff H. Rettig

Phylogeny, Classification and Floral Evolution of Water Lilies (Nymphaeaceae; Nymphaeales): A Synthesis of Non-molecular, rbcL, matK, and 18S rDNA Data

The water lilies (Nymphaeaceae) have been investigated systematically for decades because they are believed to represent an early group of angiosperms with relatively unspecialized floral organization. Although this group is small taxonomically, the relationships among genera of water lilies have eluded clarification and no single classification has become widely accepted. We present a well-corroborated phy- logeny of water lily genera that is based on agreement between non-molecular data and DNA sequences obtained from both organellar and nuclear genomes. For specific portions of the resulting phylogeny, we evaluate the support conferred by each separate data set in comparison to various combinations. This ap- proach enabled us to assess the potential benefits of further data acquisition, and also allowed us to evaluate the fundamental advantages and disadvantages of each data partition. Every data set contributed differently to the overall phylogenetic analysis and resolution of the cladogram. The 18S rDNA performed the most poorly, with homoplasious sites confounding some topological assessments in comparisons of closely related genera. However, as taxonomic distance increased, phylogenetic signal in the 18S rDNA data increased due to the expression of sequence variation in highly conserved sites. Even the 18S rDNA data were relatively congruent with the other data evaluated, and the resulting combined data analysis rendered a single maxi- mum parsimony tree with strong nodal support throughout. When floral features were evaluated using this well-corroborated phylogeny, the pleiomerous condition of water lily flowers showed several instances of secondary derivations. Although the actual morphological details of the first water lily flowers remain un- certain, it is clear that the flowers of extant water lilies do not necessarily depict the ancestral organization. Results of the phylogenetic analysis are used to encourage the adoption of an evolutionarily based classifi- cation system for water lilies.

Biogeography of discontinuously distributed hydrophytes : a molecular appraisal of intercontinental disjunctions

The extraordinarily wide distributional ranges of aquatic flowering plants have long stimulated phytogeographical discussion. Although aquatic plants occur rarely among the angiosperms, they represent a disproportionately large number of taxa with broad distributions including various intercontinental disjunctions that are manifest even at the species level. Throughout the nineteenth and early twentieth centuries, long‐range dispersal by waterfowl was the prevailing explanation for widespread aquatic plant distributions. This explanation gradually fell into disfavor as biologists raised doubts as to the ability of waterfowl to transport propagules across the extensive transoceanic distances between the continents on which an assortment of aquatic taxa now reside. During the twentieth century, the development of biogeographical displacement theory, i.e., “continental drift,” steadily began to supplant dispersal as the preferred explanation for discontinuous angiosperm distributions. Our study assesses the dispersal/displacement hypotheses from a temporal standpoint using molecular estimates of divergence time for a diverse sample of phylogenetically related aquatic taxa that exhibit discontinuous intercontinental distributions. With few exceptions, we found divergence times that are far too recent to implicate continental drift as a major determinant of discontinuous distributions in aquatic plants. We suggest that long‐distance dispersal by birds should continue to be regarded as a viable explanation for widely disjunct aquatic plant distributions, although such dispersal is likely to have involved a combination of overland as well as transoceanic migratory routes.

Genetic consequences of rarity in Aster Furcatus (asteraceae), a threatened, self-incompatible plant

Aster furcatus is a rare, self‐incompatible plant with fewer than 50 known populations throughout its range. We verified self‐incompatibility in A. furcatus by conducting experimental self‐ and cross‐pollinations and by examining seed set in a small population comprised of a single clonal genet. We examined variation at 22 electrophoretic loci in 23 populations of A. furcatus from across its range in Wisconsin, Illinois, Indiana, and Missouri. Except for two rare alleles found in single individuals in three populations, all loci but one of those examined were fixed for single alleles. The only variable locus (triosephosphate isomerase, TPI‐1) tended to exhibit genotype frequencies in Hardy‐Weinberg equilibrium or with a slight excess of heterozygotes. Although overall gene diversity was extremely low, TPI genotype frequencies were indicative of an outcrossing plant. We examined the subpopulation genetic structure among clonal plants within one Wisconsin population in greater detail. F statistics indicated that much of the genetic variation at the polymorphic TPI locus was due to differentiation among populations. We discuss the implications of self‐incompatibility and low levels of genetic variation for the evolution and conservation of Aster furcatus and other rare plants with similar breeding systems.

Studies of hybridization and chromosome number variation in aquatic angiosperms: evolutionary implications

Abstract Aquatic angiosperms are widely recognized as a biological group sharing attributes associated with adaptations to the aquatic condition. Clonal growth, high vagility of vegetative propagules, and rare to sporadic sexual reproduction are common convergences among aquatic plants, and play central, interacting roles in various evolutionary factors. In this review, two important evolutionary factors, hybridization and chromosome number variation, are discussed with respect to interactions involving clonal growth, vagility, and asexuality. Asexual reproduction emerges as a significant evolutionary catalyst allowing for the perpetuation of hybrid offspring and anomalous cytotypic variants. Inherent phenotypic plasticity in aquatic plants is difficult to discern from both hybrid individuals and cytotypic variants. Detailed studies of putative ‘hybrids’ in some groups may reveal a higher incidence of cytotypic variants at the basis of morphological differences previously attributed to hybridization.