Embriology of Miconia albicans (Sw.) (Melastomataceae), agamospermous species

Author(s)

Priscila Andressa Cortez

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Biologia
• Defense date: 2012-01-31

Examining board members

Simone de Pádua Teixeira; Eliana Regina Forni Martins; Renato Goldenberg; Daniela Guimarães Simão; Vera Lucia Scatena

• Advisor: Sandra Maria Carmello Guerreiro; Simone de Pádua Teixeira

Abstract

Most angiosperms reproduce sexually, with the production (gametogenesis) and fusion (fertilization) of the haploid gametes. However, some of them do so in an asexual way, called apomixis, a way of reproduction with the production of seeds which embryos are produced without fertilization of the egg cell. There as two main types of apomixis: one in which the embryo is formed inside the megagametophyte with somatic chromosome level (unreduced); and another one, in which there is no megagametophyte formation and the embryo is formed from ovule nucellar or tegument cells. Apomixis is often studied in Poaceae, Asteraceae, Rosaceae and Rutaceae families, probably due to its economical value. However, many studies have shown an increasing number in the tropical flora, mainly among Miconia (Melastomataceae), a paraphyletic genus with about 1050 species. Taking into account the amount of unviable pollen grains in apomictic Miconia and the importance of stamen morphology and development to the phylogenetic studies, this study aimed to (1) compare the anther and pollen grain development in Miconia albicans (apomictic species with lacks pollen grains), M. stenostachya (apomictic species with low percentage of viable pollen grains) and M. paucidens (sexual species with high percentage of viable pollen grains); (2) study the structure and ultrastructure characteristics related to the pollen grains development in M. albicans and M. stenostachya; and (3) study the relationship of the endothecium and the epidermis with the poricidal anther dehiscence mechanism in the apomicts M. albicans, M. fallax, M. leucocarpa, M. stenostachya and M. sellowiana, and in the sexual species M. chamissois, M. minutiflora, M. paucidens, M. pseudonervosa, and M. theaezans. Anthers in various stages of development were studied under light microscopy and scanning and transmission electron microscopes. The shape and the viability of mature pollen grains and the pollen tube shape were examined in fresh pollen grains obtained from mature anthers, which were submitted to hydration and "in vitro" germination testes. Tests for the main groups of chemical substances were applied to thin sections and to fresh pollen grains. M. albicans, M. stenostachya and M. paucidens have tetrasporangiated anthers, anther wall development following the "monocotyledonous" type, and pollen grains which lack reserve substances during all steps of development; in M. paucidens, crystals and proteoplasts were found inside the tapetal cell cytoplasm. The dehiscent anthers have epidermis and non-functional endothecium. The absence of meiotic figures in M. albicans may indicate that the meiosis is a rare event in this species; its mature anthers were also empty or with pollen grain-like structures, but completely empty and with abnormal pollen wall. The meiotic irregularities observed in M. stenostachya gave rise to the microspores with abnormal size (smaller or bigger than the normal ones) and often with more than four per tetrad; the null percentage of pollen grain germination in M. stenostachya showed that even the well-stained pollen grains were unviable. M. paucidens showed elongated pollen grains that are released from anther in a partially dehydrated way, being the pollen tube originated after the pollen grain hydration when in contact with the stigmatic surface. In all of the species studied, the anther dehiscence mechanism is a consequence of cell degradation in a portion that lack cuticle being, for this, independent of the endothecium layer (AU)