Androgyny of Individuals and Polygamy in Populations of Salix Myrsinifolia Salisb

Vol. 1/2, pp. 238–266 Perspectives © Gustav Fischer Verlag, 1998 in Plant Ecology, Evolution and Systematics Androgyny of individuals and polygamy in populations of Salix myrsinifolia Salisb. in the south-western part of its geographical range (NE-Poland) Janusz B. Falin´ski Geobotanical Station of Warsaw University, 17-230 Bial/owiez˙a, Poland; email: [email protected]. bial/owiez˙a.pl Abstract The aims of the study were to describe the phenomenon of androgyny (a change in the sexual behaviour of individuals) in Salix myrsinifolia, and to consider the implica- tions of androgyny for the sex structure of local populations and for the sex structure of the species across a larger geographical range. Field surveys of the sexual behaviour of individuals and populations of Salix myrsinifolia were carried out over nine years (1989–1997) in an area of 40,000 km2 of NE-Poland. More detailed studies were performed on populations in Polana Bial/owieska (the Bial/owiez˙a Clearing) in the Bial/owiez˙a Primeval Forest, in the Bial/ystok area, in the Biebrza Valley and in the experimental garden and laboratory of the Bial/owiez˙a Geobotanical Station. The bisexuality of Salix myrsinifolia is basically expressed through the development of a number of different forms of catkin that are intermediate between those that are entirely male and those that are entirely female. Flowers on bisexual catkins are fully developed, as on monosexual ones, but the male segment usually develops prior to the female one. With the exception of one form, a clear partitioning between the male and female parts of the catkin remains. This partitioning of the genders mainly takes place transversely, but in one case longitudinally. It is thus usually possible to speak of the division of a bisexual catkin into male and female sectors. In all three populations studied in detail, marked shrubs, on which bisexuality had been noted at the beginning, retained this trait throughout the 9-year observation period. However, there were changes in both the expression of bisexuality, i.e. the frequency of bisexual and monosexual catkins, and in the frequency of different patterns of bisexual catkin. Only bisexual individuals with a clear prevalence of female features retained their character. In the two larger populations studied, many originally monosexual individuals became bisexual during the study period. In the south-western part of the range of S. myrsinifolia in NE-Poland all local populations were characterized by the presence of bisexual individuals and are thus polygamous. There was a close association between the presence of S. myrsinifolia and the degree of ruderalization of habitats. The degree of polygamy in a population was also significantly correlated with ruderalization. It is concluded that androgyny and polygamy may be favoured not only by changes in environmental conditions, but also by the particular pressures to which individuals and populations of a species may be subject at the edge of its geographical range. Key words: catkin structure, Bial/owiez˙a, bisexuality, environmental conditions, polygamy, ruderalization, sex partitioning, willows Androgyny and polygamy in Salix myrsinifolia 239 Introduction Wagner 1993). Thus the issue of polygamy can only be considered with reference either Bisexuality and polygamy in the to a particular population or to a group of pop- genus Salix ulations, or to a species as a whole. Most early reports of unstable bisexuality Although the genus Salix is characterized as in Salix are for Salix myrsinifolia Salisb. (S. dioecious, the sporadic occurrence of an- nigricans Sm.; Fig. 1) and they document a drogyny and polygamy has long been ob- wide range of morphological variation. Never- served among several lowland species of wil- theless, the number of cases involved has low (Falin´ski 1997b, 1998). Androgyny nor- been small and the phenomenon is not repre- mally refers to the presence of separate male sented abundantly in herbarium collections and female flowers on the same plant. In this (see Table 3 and Discussion). It is interesting context it refers to an unstabilized form of bi- to note how earlier researchers viewed the sexuality and is a phenomenon associated phenomena of androgyny and monoecy of with the flower, the infloresence and the indi- willows (Hegelmaier 1866; Dorn 1875; vidual. In plants, polygamy means the co-oc- Hibsch 1875; Bail 1878; Potonié 1892) and currence and function of monosexual and bi- described the biology of flowers and flower- sexual individuals in a population (Schubert & ing among willows (Hampe 1840; Hegelmaier Fig. 1. Branches from two shrubs of S. myrsinifolia with bisexual catkins. The origi- nal plants grew beside a forest road in the Bial/owiez˙a Forest and were observed for seven years (1989– 1995). Branches were later transplanted to the experimental garden. The photo- graph was taken two years after transplanting (Photo J.B. Falin´ski). 240 J. B. Falin´ski 1879; Burkil 1898; Strasburger 1900). In gen- At the level of the species, polygamy in eral, androgyny was treated as a freak of na- Salix may reflect one of the following morpho- ture (lusus naturae), as an anomaly, or as a logical types (Fig. 2): teratological form (see titles of work from: 1. The individual is bisexual because some Turpin 1833; Henry & Marquart 1841; Leefe or nearly all flowers in the catkin develop 1841; Magnin 1878; von Seemen 1887). As into bisexual flowers. In some cases, the recently as 1979 Chmelar & Meusel referred pistil and stamen are fully developed and to “freaks of nature” among willows. Indeed, function properly, but in others one struc- even more recent floras and specialist identi- ture may remain relatively undeveloped. fication keys do not mention the occurrence On different shrubs or trees only some of of androgyny and polygamy in the genus (see the catkins develop in such a way while Neumann 1981; Lautenschlager 1983; the rest remain monosexual. In a local Meikle 1984; Martini & Paiero 1988). How- population, the proportion of individuals ever, Stace (1997: 230) wrote: “Bisexual with bisexual flowers is limited. catkins are not rare, especially in hybrids, in- 2. The individual is bisexual because there cluding some of those noted as female only”. has been a functional division of catkins The sporadic bisexuality of flowers, inflores- into a part with male flowers and a part cences and individuals, and hence the with female flowers. polygamy of some species of willow, was 3. The individual is bisexual because flowers thus by earlier researchers foremostly treated of both sexes appear mixed together as a departure from the monosexuality of within a catkin, but they are always mono- flowers, inflorescences or individuals, and sexual, fully-developed and properly func- from the dioecy which characterizes the tional (Fig. 1). genus as a whole. General descriptions of the development Linné (1753) advocated a “taxonomic ap- and variability of bisexual flowers (i.e. type 1) proach” to exceptional phenomena. For ex- have been given by Rainio (1927), and later ample, some 20 years after the publication of by Maljutina (1972, 1973a, 1973b, 1974) and “Systema Plantarum” in 1733 he described Maljutina & Maljutin (1972) mainly for Salix Salix hermaphrodita (as a form of S. pentan- myrsinifolia and S. cinerea and their hybrids. dra). Following this lead, during the 19th and In contrast, there appears to be no literature early 20th centuries different departures from referring to the development of different kinds dioecy had been described as separate of catkins (types 2 and 3), and the presence of forms, variants and subspecies of what were bisexual and polygamous individuals in such fundamentally dioecious species. Several ex- populations apart from occasional reports of amples of this approach can be found in the “aberrant taxonomic” forms and “anomalies” work of Rainio (1927). (see above). The study by Elmqvist et al. In the present study dioecy is treated as (1988) devoted to the reproductive biology of one of eleven biological features which char- S. myrsinifolia did not address the issues of acterize woody pioneer species (Falin´ski bisexuality and polygamy in this species. 1980a, 1980b, 1991, 1996, 1997a, 1997b), Based on long-term observations of thus augmenting the list established by other marked willows and examination of herbarium authors, notably Iversen (1973). Dioecy is a materials, I have come to the conclusion that feature of all species of Salix and hence of polygamy as a result of the development of one of the largest genera of woody plants, one of various forms of bisexuality in individual with some 300 species according to Rehder willows has recently become much more fre- (1940), Skvorcov (1968) and Chmelar & quent in Poland. It is sometimes even com- Meusel (1979), or 500 according to Rechin- mon and has persisted for many years (Fa- ger (1958) and Krüssmann (1978). The lin´ski 1996, 1997b). This conclusion that poly- genus also exhibits entomogamy and gamy is increasing in some populations of wil- anemochory, both features are regarded as lows relates to the spread of bisexuality of the characteristic of pioneer plants (Falin´ski second and third types in S. myrsinifolia. In 1980a, 1996). In comparison, Populus, also a this monograph I describe the phenomenon member of the Salicaceae, has only 40 chiefly in relation to the population structure species. These are also dioecious, though and geographical range of this species, and anemogamic and anemochoric, and bisexu- the environmental conditions where it occurs. I ality occurs rather rarely. have also observed the same form of bisexu- Androgyny and polygamy in Salix myrsinifolia 241 Fig. 2. Patterns in the partitioning of the sexes on bisexual catkins of S. myrsinifolia (twelve types) distinguished at the beginning of the study and used in de- scribing the bisexuality of shrubs in the field (cf.

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