Western Connecticut State University WestCollections: digitalcommons@wcsu Department of Biology & Environmental Sciences Faculty Papers Department of Biology & Environmental Sciences 12-1996 Evolution of Aquatic Angiosperm Reproductive Systems Tom Philbrick PhD Western Connecticut State University, [email protected] Donald E. Les PhD University of Connecticut Follow this and additional works at: https://repository.wcsu.edu/biologypaper Part of the Plant Sciences Commons Recommended Citation Philbrick, Tom PhD and Les, Donald E. PhD, "Evolution of Aquatic Angiosperm Reproductive Systems" (1996). Department of Biology & Environmental Sciences Faculty Papers. 5. https://repository.wcsu.edu/biologypaper/5 Evolution of Aquatic Angiosperm Reproductive Systems What is the balance between sexual and asexual reproduction in aquatic angiosperms? C. Thomas Philbrick and Donald H. Les s angiosperms diversified and greater chemical and thermal stabil- flourished in terrestrial habi- Aquatic plants are an ity than air, and it buffers against (or Atats, some species ultimately even precludes) many types of cata- colonized freshwater or marine en- strophic disturbance that plague ter- vironments and became aquatic. extremely heterogeneous restrial habitats, such as rapid tem- Aquatic plants are species that per- assemblage of species perature changes, fires, floods, and petuate their life cycle in still or strong winds. At higher latitudes, flowing water, or on inundated or that survive in similar seasonal stability of aquatic habitats noninundared hydric soils. Aquatic is faithfully maintained by the den- angiosperms inhabit oceans, lakes, habitats but as a result of sity of water, which is greatest (de- rivers, and wetlands. pending on salinity) at approximately The transition to an aquatic life fundamentally different 4°C (Wetzel 1975). Thus, even in has been achieved by only 2% of the the coldest temperatures, lake and approximately 350,000 angiosperm evolutionary pathways river bottoms typically remain ice species (Cook 1990). Nonetheless, free. the evolutionary invasion of aquatic with specific growth forms: emersed Coastline and freshwater shore environments by terrestrial an- from the water, free-floating, float- aquatic habitats have been viewed giosperms is estimated to represent ing-leaved, or submersed. These cat- as inherently unstable (Laushman 50-100 independent events (Cook egories represent different degrees 1993) due to erosional processes, 1990). Although aquatic plants are of adaptation to aquatic life and are tidal fluctuations, and wave dynam- typically discussed as a unified bio- widely convergent among aquatic ics. However, habitat stability should logical group, the ways that species angiosperms. be evaluated not only in terms of have evolved to life in the aquatic As in terrestrial plants, reproduc- characteristic short-term variation, milieu are as diverse as the different tion in water plants consists of both but also over the course of longer, evolutionary lineages that became sexual and asexual mechanisms. evolutionarily significant time aquatic (Hutchinson 1975, Scul- Sexual reproduction (the chief source frames. In this sense, stability re- thorpe 1967). Reproductive and of hereditary variation via genetic flects the consistent expression of other life-history traits of aquatic recombination) in plants is consid- predictable habitat characteristics angiosperms are closely associated ered to be advantageous in changing over long time periods. In essence, or heterogeneous environments, and aquatic habitats may be quite vari- asexual reproduction (which per- able, yet vary in a similar, predict- able fashion through time. The an- C. Thomas Philbrick is an associate petuates genetic uniformity) is con- professor in the Department of Biologi- sidered to be more successful in stable giosperm family Podostemaceae cal and Environmental Sciences, West- or uniform habitats (Grant 1981, (riverweeds) illustrates this concept ern Connecticut State University, Williams 1975). Consequently, the well. Riverweeds grow tenaciously Danhury, CT 06810. Donald H. Les is evolution of aquatic plant reproduc- attached to rocks in tropical river an associate professor in the Depart- tive systems should reflect the rela- rapids and waterfalls. Although the ment of Ecology and Evolutionary Biol- tive stability of their habitats. rushing current makes this habitat ogy, University of Connecticut, Storrs, unstable ecologically, the seasonally CT 06169-3042. The authors share re- A vast assortment of freshwater high and low water levels make it a search interests in the systematics and and marine environments exists. predictable habitat in which river- evolution of aquatic flowering plants. Nevertheless, aquatic habitats tend weeds flourish (Philbrick and Novelo © 1996 American Institute of Biologi- to be stable (Hartog 1970, Sculthorpe 1995). cal Sciences. 1967, Tiffney 1981). Water exhibits December 1996 813 No aquatic habitat is absolutely milfoil {Myriophyllum spicatum, stable. Factors such as continental Haloragaceae) have spread over vast drift have lead to drastic ecological areas by asexual means. Field stud- changes in coastal marine environ- ies (Les 1990) indicate that plots ments. Cultural eutrophication and planted with small fragments of pollution can rapidly alter the trophic water milfoil can reach carrying ca- status of aquatic habitats. The spo- pacity in only 16 months (Figure 1). radic outbreak of pathogens, such as Such results express the futility of the agent responsible for the devas- 10 13 14 16 1B 20 23 34 ZE 37 control efforts if aquatic weed intro- tating wasting disease of the seagrass ductions are not recognized, and plants eradicated, immediately after Zostera marina (Zosteraceae; Muchl- Figure 1. Asexual reproduction in stein et al. 1991), is yet another aquatic plants occurs rapidly. Biomass initial colonization. aspect of instability in aquatic envi- (grams of dry weight) measured in 2 m Most aquatic plants are not ronments. X 2 m field plots planted initially with troublesome but possess mechanisms In evolutionary time frames, 100 small fragments of Eurasian water for asexual reproduction similar to aquatic habitats represent a mosaic milfoil (Myriophyllum spicatum). those of their weedy counterparts. of both stable and unstable condi- Within 16 months, vegetative growth Many common names such as wa- had reached maximum biomass levels terweed, pondweed, and riverweed tions to which complex adaptation (carrying capacity). Biomass had more has been necessary. Recalling the than doubled during the first four-month are unwarranted but bave probably paradigm for the evolution of asexual growing season (from Les et al. 1988). originated because of tbe tendency and sexual reproductive systems, it for water plants to grow in luxuriant is evident that both systems should beds formed by vigorous vegetative retain important functions in the etatively produced progeny) are not growtb. Some native aquatic plants majority of water plant species. In always identical genetically to the are actually more productive than this article, we discuss possible evo- parent (see below). In any case, they introduced weedy species but have a lutionary factors to account for the represent a legitimate example of less effective vegetative growtb ar- balance between sexual and asexual reproduction in which discrete, new chitecture. For example, experiments reproduction that is maintained in individuals are produced and dis- in wbicb vegetative fragments from aquatic angiosperms. persed. botb a native pondweed and intro- duced milfoil species were planted Asexual reproduction is impor- simultaneously sbow greater bio- Asexual reproduction tant in the establishment, growth, mass productivity in the native spe- and maintenance of aquatic plant cies (Table 1; Les et al. 1988). Addi- Asexual reproduction includes both populations. For example, in weedy tional experiments have furtber seed production without fertiliza- aquatic plants, most emergent spe- indicated no evidence of competi- tion (agamospermy) and vegetative cies disperse by sexual propagules, tion between these species under reproduction. Because the extent of whereas floating and submersed spe- normal environmental conditions agamospermy among aquatic plants cies disperse vegetatively (Cook (Les et al. 1988). Elevated nutrients is poorly understood (Les 1988a), 1993, Spencer and Bowes 1993). resulted in tbe accelerated growtb of we limit our discussion to vegetative Nevertheless, the principal means of both species (Les 1990), but milfoil reproduction, which is often assumed population increase for all three biomass was mostly allocated to pro- to be the dominant mode of repro- growth forms is by vegetative repro- duce long, vertical sboots, wbereas duction in water plants (Hutchinson duction (Spencer and Bowes 1993). much pondweed biomass was allo- 1975, Sculthorpe 1967). Abraham- Certainly, the ease and rapidity by cated to borizontal rhizomes (Table son (1980) considered that geneti- which aquatic weeds spread through- 1). Rapid vertical growtb under en- cally identical offspring render the out nonindigenous regions attests to banced nutrient regimes enables mil- process of vegetative reproduction the efficiency of vegetative repro- foil to quickly grow to tbe water more similar to growth (increase in duction. surface, where it shades native plants, size of an individual) than to repro- Nuisance aquatic weeds such as indirectly causing their decline. duction (increase