Tristyly in Eichhornia crassipes (Mart.) Solms (Water Hyacinth)

Recent workers have considered Eichhornia crassipes (Mart.) Solms to be distylous since the short-styled form had not been reliably recorded. This "missing" form was recently found in the Lower Amazon, and the species should now be considered tristylous. Populations of E. crassipes in the native and adventive range are characterized by the predominance of a single style form, a condition atypical for heterostylous species. In most of the adventive range of E. crassipes the midstyled form predominates, with long-styled forms occurring less frequently. The short-styled form is apparently restricted to tropical South America. Associated with floral trimorphism in E. crassipes is a weak pollen trimorphism. The three floral forms of E. crassipes are highly self-compatible. Within style forms there is tittle difference in seed production between self and legitimate pollinations. During a study of populations in the Lower Amazon, the major insect visitor to flowers of E. crassipes was the bee Ancyloscelis gigas. Of 1208 flowers sampled from four monomorphic populations, 67.5 percent produced seed. In the majority of populations of E. crassipes tristyly does not function as an outcrossing mechanism due to population structure. THE WATER HYACINTH Eichhornia crassipes (Mart.) Solms is a heterostylous, free-floating aquatic member of the monocotyledonous family Pontederiaceae. During the past century it has spread from its native rnge in the New World tropics to achieve a world-wide distribution (Little 1965, Sculthorpe 1967). Man has played a major role in facilitating this spread, both deliberately (Backer 1951, Vietmeyer 1975) and unintentionally. Today E. crassipes is considered one of the most dangerous aquatic weeds, and considerable annual expenditure results from attempts at eradicating its extensive mats from reservoirs, canals, and rivers. Due to the economic importance of E. crassipes a large body of literature, concerned with various phases of its life history, has accumulated in recent years (see Penfound and Earle 1948, and Bock 1966 for reviews). Despite this attention, little is known about seed reproduction in Eichhornia, and, in particular, the type and manner of functioning of its breeding system. In 1957, speakers at a symposium on the problem of the weed in Africa deplored the shortage of information on seed reproduction (Anon. 1957). Due to its extensive vegetative growth (see Penfound and Earle 1948) workers have tended to minimize the importance of sexual reproduction in E. crassipes (Baker 1965, Mulcahy 1975). Sexual reproduction has not been reported in certain parts of the adventive range, such as California (Baker 1965, Bock 1966). However, in tropical regions seed reproduction can occur and is important in aiding the spread of E. crssipes (Ridley 1930, Haigh 1936, Pettet 1964). Most workers have studied E. crassipes in its adventive range, and very little is known of the reproductive biology of populations in the native range (Sculthorpe 1967). The purpose of this paper is to describe the floral trimorphism and breeding system of E. crassipes and to report the findings of a study of populations in the Lower Amazon Basin where E. crassipes is considered to be native (Schwartz 1927). TRISTYLY IN Eichhornia crassipes The Pontederiaceae is one of three flowering plant families which possess the form of floiral heteroniorphism known as tristyly. Tristylous species are characterized by plants of three types differing in floral morphology. The floral forms are: (1) flowers with long styles and two anther levels below the stigmas (mid and short); (2) mid-styled flowers with one set of anthers above the stigmas (long) and one set below the stigmas (short); (3) short-styled flowers with anthers at two levels above the stigmas (mid and long). Tristyly and the more widespread distyly are floral mechanisms which promote outcrossing between floral forms (Darwin 1877). Ever since Fritz Muller (1871, 1883) reported midand long-styled forms of E. crassipes in southern Brazil, there has been disagreement and confusion as to whether E. crassipes is distylous or tristylous. The first report of tristyly was made in the English edition (1883) of "Die Befruchtung der Blumen durch Insekten" by Herman Muller (1873). However, this report of tristyly is the result of an apparent mistranslation by D'Arcy W. Thompson of the original German edition, where no mention is made of the short-styled 230 BIOTROPICA 9(4): 230-238 1977 1 Present address: Department of Botany, University of Toronto, Toronto, Canada M5S lAl. This content downloaded from 157.55. 39.138 on Thu, 19 May 2016 05:03:43 UTC All use subject to http://about.jstor. org/terms form. Although H. Mdller and later Darwin (1877) stated that E. crassipes was probably trimorphic, these statements were based on the assumption that the missing short-styled form would be found (Bock 1966). Since then there have been erroneous (Bir Bahadur 1968) and unsubstantiated (Hauman-Merck 1913) reports of the short-styled form, statements that E. crassipes is tristylous (Agharkar and Banerji 1930, Tackholm and Drar 1950, Willis 1951), as well as an incorrect report by Wild (1961) that the styles of this species are "sometimes of three different lengths in the same plant. " In reviewing heterostyly in E. crassipes, both FranSois (1964) and Ornduff (1966, 1974) considered that there was insufficient evidence for the existence of the shortstyled form, and recent workers have considered the species to be presently distylous (Bock 1966, Faegri and van der Pijl 1971, Mulcahy 1975). STYLE FORM DISTRIBUTION AND POPULATION STRUCTURE. -The predominance of a single style form of E. crassipes in a particular geographical area has been noted by previous workers (MiUller 1883, Haigh 1936, Penfound and Earle 1948, Backer 1951, Haldane 1960, Bock 1966, Sculthorpe 1967, Mulcahy 1975, Tag el Seed and Obeid 1975). In most of the adventive range of the species, mid-styled plants are most commonly encountered, with long-styled plants occurring occasionally or not at all (fig. 1). There are no documented reports of short-styled plants of E. crassipes in the adventive range nor of populations containing equal proportions of style forms, even though this is a typical condition in many hetero-