Dioecious Habit Research Articles

Growth of male and female Araucaria araucana trees respond differently to regional mast events, creating sex-specific patterns in their tree-ring chronologies

Araucaria araucana is a dioecious evergreen conifer endemic from temperate forests of south Argentina and Chile. It is a long-lived species (maximum age > 1000 years), and it presents a high potential for tree-ring based climate reconstructions. However, the species’ dioecious habit can result in distinct sex-specific growth patterns, which introduce novel challenges in the interpretation of tree-ring chronology variations. We used a network of 10 tree-ring chronologies from northwest Patagonia (Argentina) to analyze sex-specific growth patterns in A. araucana and, for the first time, demonstrate that they result from the contrasting responses of ring width index of male (RWImale) and female (RWIfemale) trees to regional mast events (years with high seed production). During the year of seed maturation and seed dispersal, the growth of females was strongly and significantly reduced, while a growth response of similar magnitude was found in male trees in the previous year, corresponding to the year of pollination. We interpret these growth responses as representing contrasting allocation shifts between growth and reproduction in males and females. The sex-specific growth responses associated with mast events resulted in a particularly strong and distinct signal in a RWImale–RWIfemale chronology. Male and female tree-ring chronologies shared a strong common signal, and responded similarly (but not exactly) to broad-scale climatic conditions in the growing season. Our results indicate that sex-specific tree-ring chronologies can be used to isolate mast events in A. araucana chronologies, providing an opportunity to develop unique multi-century reconstructions of large mast events, and improve dendroclimatic calibration for this species.

Rediscovery and amended descriptions of Begonia kingdon-wardii (Begoniaceae) from North Myanmar.

Begonia kingdon-wardii Tebbitt was rediscovered in 2014 from Myanmar after 67 years based on its last collection in 1937. Its previously unknown female flower and inaccurate morphology of leaf and ovary have been additionally described. This species belongs to Begonia sect. Sphenanthera (Hassk.) Warb. due to its dioecious habit, 3-locular ovary, berry fruits and thick placenta segments. Morphologically, it is similar to Begonia gulinqingensis S. H. Huang & Y. M. Shui in the leaf shape, placentation and fruit shape, but different in its dioecious plants, pliciform leaves, two-petalled female flowers and berry fruits. The rediscovery of this amazing living species will attract significant interest for scientific research and horticultural application.

Identification of Cambodian Gnetum (Gnetaceae, Gnetales) species by DNA barcoding

Gnetum (Gnetaceae, Gnetales) is a gymnosperm genus with ca. 35 species distributed in tropical forests around the world. Due to its dioecious habit and lack of diagnostic characters from vegetative tissue, the identification of Gnetum species is not easy without seeds or reproductive structures. To identify and verify their phylogenetic positions, we applied DNA barcoding to Cambodian Gnetum collections gathered between 2010 and 2015, with previously designed cp matK gene primers. We newly sequenced partial matK sequences from 72 Gnetum collections, 43 out of 72 from Cambodia, and analyzed 115 Gnetum accessions using the neighbor-joining method. The resulting neighbor-joining tree categorized Cambodian Gnetum samples into three clades of species: G. macrostachyum, G. montanum, and G. aff. gracilipes. The recognition of G. aff. gracilipes in Cambodia is reported here for the first time. Taxonomic information for the three recognized Cambodian Gnetum species is provided and the benefits of the taxonomic reevaluation assisted by DNA barcoding are emphasized in this work.

Lycium strandveldense (Solanaceae), a new species from the western coast of South Africa

A new species, L. strandveldense, from the western coastal region of the Western and Northern Cape Provinces, is described and figured. L. strandveldense, which is functionally dioecious, resembles the bisexual L. afrum L., a species also found along the western coast. Because of the near similarity in leaf and floral characteristics, specimens of L. strandveldense have in the past been regarded as atypical forms of L. afrum. This new species is distinguished by its functionally dioecious habit, relatively short, narrowly oblong, bright green leaves instead of the long, linear, glaucous leaves of L. afrum, corolla dark purple instead of claret coloured, corolla tube shorter than in L. afrum, stamens of the male flowers slightly exserted from corolla mouth in contrast to the included stamens of L. afrum, and finally a smaller ellipsoid red berry instead of the large spherical black berry of L. afrum.

Lycium gariepense (Solanaceae), a new species from South Africa and Namibia

Lycium gariepense , a new species from South Africa and Namibia, is described and figured. Its centre of dispersal seems to be the lower Orange River Valley of the Richtersveld and south-western Namibia. The new species is characterised by a functionally dioecious habit, feathery appearance, intricately branched and spreading growth form, much-branched, needle-like thorns, and flowers in which the staminal anthers are included in the corolla tube or protrude only slightly from the corolla mouth. This new species resembles L . bosciifolium Schinz and L . horridum Thunb.

Conservation of montane willow scrub in Scotland

Summary The community known as montane willow scrub has been almost totally destroyed in Scotland by human activities. The species of montane willows are not regarded as threatened by the conventional criteria, but the continued effects of grazing, combined with their sparse distribution in suboptimal habitat and dioecious habit, suggest that the long term threat to some species may have been underestimated. Conservation measures are needed urgently if we are to restore areas of montane willow scrub. It will be necessary to use fencing to provide protection from grazing, and also it may well be necessary to do some planting to provide enough individuals for effective pollination and regeneration to occur.

BREEDING SYSTEMS AND DISPERSAL-RELATED MATERNAL REPRODUCTIVE EFFORT OF SOUTHERN SPANISH BIRD-DISPERSED PLANTS.

Dioecious plants, characterized by the occurrence of separate male and female individuals, represent a relatively unimportant fraction of the total world's flora (Bawa, 1980). Nevertheless, owing to the nonrandom distribution of the dioecious habit among life forms, successional stages and habitat types, dioecy may become a prominent feature of some regional or taxonomic plant assemblages (Ashton, 1969; Bawa, 1974; Bawa and Opler, 1975; Bawa, 1980; Conn et al., 1980; Freeman et al., 1980; Givnish, 1980). The dioecious condition has been found to be relatively overrepresented in later successional stages of tropical vegetation (Opler et al., 1980) and among trees and shrubs (Bawa, 1980; Conn et al., 1980; Freeman et al., 1980), vertebrate-dispersed plants (Bawa, 1980; Givnish, 1980), small-flowered tropical species (Bawa and Opler, 1975; Opler et al., 1975), wind-pollinated temperate plants (Freeman et al., 1980), and tropical species pollinated by small opportunistic insects (Opler et al., 1975; Bawa, 1980). Although promoting outcrossing has been traditionally suggested as the principal selective pressure underlying the evolution of dioecy (see recent reviews by Willson, 1979; Bawa, 1980; Givnish, 1980), some of the ecological correlates of the dioecious habit have recently served to suggest alternative and/or complementary explanations to its evolution (Freeman et al., 1976; Willson, 1979; Bawa, 1980; Freeman et al., 1980; Givnish, 1980; Cox, 1981). In particular, the strong association found between dioecy and seed dispersal by vertebrates has lent support to the notion of this breeding system having evolved among these plants in response to disperser-mediated, intrasexual competition pressures favoring a disequilibrium in the allocation of resources to male and female functions (Bawa, 1980; Givnish, 1980; but see Thomson and Barrett, 1981). It was assumed by the lastmentioned authors that vertebrate dispersers will select for higher dispersal-related maternal expenditure, thus ultimately favoring the spread of dioecy. In this context, it is of interest to compare female investment in disperser-attracting structures shown by dioecious and nondioecious vertebrate-dispersed plants. Increased returns in fitness with increasing maternal absolute reproductive effort were considered by Bawa (1980) and Givnish (1980) in relation to the size of fruit crops only, assuming that plants with larger fruit crops have greater attractiveness to dispersers (Howe and Estabrook, 1977; Howe and Vande Kerckhove, 1979; Howe, 1980). Nevertheless, crop size is just a single component of the total female reproductive investment associated with vertebrate-dispersal. The amount of flesh (the food reward which attracts dispersers to fruits; Snow, 1971; McKey, 1975) per fruit, and the energy and nutrients per weight unit of flesh, are two further important aspects of a plant's fruiting pattern which influence fruit consumption by frugivores and hence, presumably, the fitness of the plant. At an intraspecific level, frugivores have been shown to favor plant phenotypes with a higher flesh content per fruit (Howe and Vande Kerckhove, 1980) and, among species, dispersers seem to prefer those with the most nutritious fruits and/or those with higher flesh/seed ratios (Herrera, 1981a, 1981b; Jordano and Her-

INFLUENCE OF ENVIRONMENT ON THE FLORAL SEX RATIO OF MONOECIOUS PLANTS.

Both theoretical and empirical data suggest that sessile, simultaneous hermaphroditic individuals growing in patchy environments may experience unequal reproductive success via male and female gametes depending on whether they occupy favorable or marginal sites (Schaffner, 1935; Heslop-Harrison, 1972; Freeman et al., 1976, 1980; Charnov and Bull, 1977; Horovitz, 1978; Charnov, 1979a, 1979b; Willson, 1979). Under such conditions, differential tendencies toward maleness and femaleness should be selected for among individuals confined to favorable or unfavorable patches. Further, simultaneously hermaphroditic individuals should display large variations in the sex ratio of flowers on different individuals (Willson, 1979). Such variation might be expected to be controlled by both genetic and environmental cues, if the patches are small and/or the quality of any given patch varies significantly between breeding seasons (Heslop-Harrison, 1972; Freeman et al. 1976; Charnov and Bull, 1977; Frankel and Galun, 1977; Willson, 1979). In 1975, we began a series of studies to determine if the sex ratios of dioecious species differed on sites of differing water availability. We demonstrated that, in five dioecious species, males were proportionately more abundant on xeric sites than on mesic sites, while the females were over-represented on mesic sites (Freeman et al., 1976). Others have made similar observations (Pickett, 1915; Davey and Gibson, 1917; Schaffner, 1922; Lysova and Khizhnyak, 1975; Richards, 1975). We proposed that this pattern was due to disruptive selection acting on the differential success of male and female gametes on sites of differing quality. It seems likely that such forces may have been important in the evolution of the dioecious habit, at least in arid environments. Since disruptive selection appeared to alter the relative success of the male and female functions of dioecious species occupying a local array of sites that differed in the amount of available soil moisture during much of the growing season, it seemed likely that monoecious plants would also differentially emphasize maleness and femaleness on sites of unequal quality. In this note we examine the floral sex ratios of three monoecious species, and how these ratios differ on sites of differing water availability. Since our data (unpubl.) show that most temperate monoecious species are wind pollinated, we have restricted our studies to wind pollinated species.

Ecology of plant dioecy in the intermountain region of Western North America and California.

Data from 44 plant communities of the arid zone of western North America and from the entire California flora suggest that the dioecious habit (separate sexed individuals) is best developed among wind pollinated woody plants. A rationale is presented for the ways in which wind pollination and large plant size have favored the development of the dioecious habit. Based upon the patterns examined, it is concluded that inbreeding depression alone is insufficient to account for the evolution of the dioecious habit in many temperate species.

Cytology of <i>Trichosanthes palmata</i> Roxb

The cytology of male and female plants of Trichosanthes palmata Roxb. has been studied. Both plants showed 22 chromosomes in root tip cells. Meiotic studies on the male plants revealed the presence of 11 bivalents in pollen mother cells. These plants are believed to be diploids on the basic number x=11. The present finding of a diploid and earlier report of a tetraploid reveal the existence of intra specific polyploidy in T. palmata. Critical examination of karyotypes in the male and female plants did not reveal the presence of morphologically distinct sex chromosomes associated with the dioecious habit.

RECENT MIGRATION AND EVOLUTION OF THE DIOECIOUS AMARANTHS

The genus Amaranthus is notable mainly because of the success of many of its members as fellow-travelers of mankind. Sometimes deliberately cultivated as grain crops, green vegetables, dye plants, or ornamentals, more often unintentionally encouraged weeds, many amaranth species have spread and diversified with the advance of artificial habitats on every continent. The part of the genus considered here is a group of ten related North American species, sharply set off from all other amaranths of the world by their dioecious habit. Descriptions of the morphology and geography of each species, documented by specimen citations, with other information appropriate to a conventional taxonomic revision, are given elsewhere (Sauer, 1955). The present paper is an attempt to reconstruct some of their history. This group is more open to such reconstruction than most of the genus because its distribution and heredity were less radically reshaped by ancient man. None of the dioecious amaranths have become domesticated plants or pandemic weeds. Yet, they have also been increasing in geographic and genetic ranges in response to civilization. Tracing their comparatively plain case histories may be a fair approach to understanding the behavior of the whole genus.

WEED CONTROL: APPLIED BOTANY

WEED CONTROL: APPLIED BOTANY

Um estudo citológico comparativo de Thalictrum e Ilex

1. New chromosome numbers have been determined tor the following species from material collected in various counties of North Carolina, U . S . A. Thalictrum clavatum n = 7 Th. dioicum n = 14 Th. polygamum n = 42 Th. coriaceum n = 70 Ilex decidua n = 20 I. monticola n = 20 I. vomitoria n = 20 I. opaca n = 18 I. verticillata n = 11. New chromosome numbers have been determined tor the following species from material collected in various counties of North Carolina, U.S.A. Thalictrum clavatum n = 7 Th. dioicum n = 14 Th. polygamum n = 42 Th. coriaceum n = 70 Ilex decidua n = 20 I. monticola n = 20 I. vomitoria n = 20 I. opaca n = 18 I. verticillata n = 18 2. The genus Thalictrum has been rather well investigated cytologically but the genus Ilex has not yet received the attention it merits, and offers cytologists in North, Central, and South America an opportunity ro study an extensive genus which has been, and probably is, undergoing interespecific hybridization while a shift from the hermaphroditic to the unisexual habit is also well established. These two great evolutionary factors, hybridization and the advent of the dioecious condition, appear to stimulate divergent lines of meiotic behavior depending upon whether or not they appear in the phylogeny of a race simultaneously or successively, and in the case of the latter, which came first. The genus Thalictrum was used as a basis for comparison with the situation as found in the eastern North American forms of Hex with the following conclusions. a. The genus Thalictrum is vastly polyploid and meiotically regular. This condition is thought to be the result of previous hybridization of the species before the advent of the dioecious habit. b. The genus Ilex is, as far as research has gone, negatively aneuploid and frequently irregular meiotically. This condition is thought to be the result of previous hybridization of the species after , or no more than simultaneously with, the advent oi the dioecious habit. Heterochromosome Complex Division in first seen Choromatin ejected Other diagnostic hybrid characteristics Species Type of Chromosome I.. decidua Medium Homeotypic One choromatid during second anaphase Laggards anaphase bridges I. monticola. Smallest member of complement Heterotypic One chormatid during first anaphase. Left in cytoplasm Lagging Precocious chorm I. opaca Medium Heterotypic Whole chorm. One chromatid sometimes accepted by complement in second div. Lagging chrom I. verticillata Two types. One tripartite X-- X1Y2 small Other medium Heterotypic Yes. Degenerates during first telophase. Yes Degenerates during first telophase. I. vomitoria Medium sized tripartite complex Heterotypic None found A number of laggards dur-first division 3. Heterochromosomes were found in microsporogenesis of all five species of Ilex, along with other meiotic peculiarities of suggested hybrid origin, but no two were alike. Behavior of the heterochromosomes was not found to be consistent and in approximately 5 0% of the PMCs. meiosis was normal. Findings are summarized in following table.8 4. In view of the generally irabie chromosome number for the five species of Ilex investigated, only such pollen as arises from PMCs. which possess the normal meiosis may produce the gametes which effect fertilization. This presents the problem of explaining how the ability to produce meiotic abnormalities is inherited through cells which dcnot exhibit it or through the female side and its significance on the whole question of the evolution of sex chromosomes from various types of heterochromosomes. 5. A comparison of the cytology or species of Thalictrum and Ilex adds support to the author's earlier contention that so-called sex chromosomes in higher plants have their origin in special types of heterochromosomes which in turn owe their appearance to meiotic disturbances induced by previous hybridization of the species and preserved through the more or less simultaneous advent of the dioecious habit. In the author'- opinion, the genus Ilex offers an opportunity to increase our knowledge of what occur? during the stage intermediate between autosome and the specialized types of sex-linked heterochromosomes.