Botanical Journal of the Linnean Society, 2004, 144, 343–349. With 11 figures © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society, 2004, 144, 343–349 343 Blackwell Science, LtdOxford, UKBOJBotanical Journal of the Linnean Society0024-4074The Linnean Society of London, 2003? 2003 144? 343349 Original Article REPRODUCTIVE BIOLOGY OF MIMOSA J. G. SEIJO and V. G. SOLÍS NEFFA *Corresponding author. E-mail: seijo@agr.unne.edu.ar The cytological origin of the polyads and their significance in the reproductive biology of Mimosa bimucronata J. GUILLERMO SEIJO 1 * and VIVIANA G. SOLÍS NEFFA 2 1 Instituto de Botánica del Nordeste (UNNE – CONICET), Casilla de Correo 209, 3400 Corrientes, Argentina 2 Facultad de Ciencias Agrarias (UNNE), Instituto de Botánica del Nordeste (UNNE – CONICET), Casilla de Correo 209, 3400 Corrientes, Argentina Received April 2003; accepted for publication July 2003 The mechanism of polyad formation in Mimosa bimucronata (DC.) Kuntze is described and its significance in the reproductive biology is discussed. In this species c. 500 complexes of two coupled pollen mother cells (PMCs) per pol- len sac are formed after the last mitotic division of the sporogenous tissue. All the pollen grains of each complex remain attached and form a bitetrad. This mechanism leads to the formation of many polyads per pollen sac. On this basis, we propose that in Mimosa the number of pollen grains per polyad (eight or 16) is determined by the number of cells that remain attached after the last mitotic divisions of the sporogenous tissue. Concerning the biological sig- nificance of the polyads in M. bimucronata, we find that the number of pollen grains per polyad is correlated with the number of ovules per ovary, and that the internal dimensions of the pit-shaped stigma are sufficient just for one polyad to be localized in it. Based on the number of polyads per flower this species has a high pollen/ovule ratio char- acteristic of xenogamous plants, in opposition to that expected for plants with composite pollen dispersal units. In contrast, it has low pollination efficiency, low fruit set, a 1/1 relationship between the pollen grains of a polyad and the number of ovules per ovary, and high seed set per fruit. These facts support the hypothesis that the occurrence of polyads in M. bimucronata would be an adaptation helping to ensure high seed set following a single cross pol- lination event. © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society, 2004, 144, 343– 349. ADDITIONAL KEYWORDS: bitetrad – gametogenesis – mimosoids. INTRODUCTION Compound pollen units occur in about 15% of families of flowering plants (Kenrick & Knox, 1982), and are very frequent in the genera of Mimosoidae (Legumin- osae). The compound units of mimosoid species are constituted by the permanent assemblage of more-or- less independent grains. These compound units have been diversely named as polyads, bitetrads and tet- rads according to the number of pollen grains that remain attached (Van Campo & Guinet, 1961; Sorsa, 1969). The possession of polyads was assumed to confer a selective advantage in reproduction, as it would pro- vide an efficient way of pollen dispersal by insects (Krees, 1981; Kenrick & Knox, 1982). However, Wyatt, Broyles & Lipow (2000) considered this hypothesis unlikely, because it would be difficult to understand why composite pollen grains should have originated as a solution to the general problem of low pollination efficiency only in few groups of dicots. According to these authors, pollen agglutination may be an adap- tation that evolved in order to avoid mixed pollen loads. A mixed load of self and cross-pollen has been reported as especially wasteful in groups such as the mimosoids and Asclepias, because these groups are known to possess post-zygotic self-incompatibility sys- tems (Lipow & Wyatt, 2000; Wyatt et al., 2000). It was also argued that pollen agglutination evolved in concert with ovule number and that plants with agglutinated pollen, such as Asclepiadaceae and Mimosoideae, constitute notable exceptions to the Downloaded from https://academic.oup.com/botlinnean/article-abstract/144/3/343/2420291 by guest on 21 May 2020