Gametes with the somatic chromosome number: mechanisms of their formation and role in the evolution of autopolyploid plants

Abstract

summary The production of 2n gametes in plants, i.e. gametes with a somatic chromosome number, is considered to be the dominant process involved in the origin of polyploid plants. In this review, we provide a synthesis of current knowledge concerning the production of 2n gametes. Firstly, we describe the different methods used to detect and quantify the production of 2n gametes in plants, which include morphological and flow cytometry screening of the occurrence of 2n pollen, the analysis of crosses among diploid and tetraploid parents and the instigation of micro‐and mega‐sporogenesis. Secondly, the high level of inter‐ and infra‐specific variation in 2n gametes production is described. Thirdly, the various cytological anomalies responsible for the production of 2n gametes are reviewed, with particular reference to the relative genetic consequences of the first and second restitution divisions that give rise to 2n gametes. Fourthly, the significance of 2n gametes in crop plant improvement is discussed, in relation to somatic chromosome doubling to obtain new polyploid varieties. In particular, we compare the genetic and yield consequences of methods based on unilateral and bilateral sexual polyploidization. Finally, we outline how knowledge of the variety of mechanisms involved in 2n gamete production have increased our understanding of the evolutionary significance of polyploidy and the population biology of polyploid plants.

Aberrant Classopollis pollen reveals evidence for unreduced (2 n ) pollen in the conifer family Cheirolepidiaceae during the Triassic–Jurassic transition

“…Principally, the observed pattern can be ascribed to the formation of anucleate, haploid and unreduced 2n pollen. In modern plants, pollen size is related to the number of chromosomes with increasing pollen volume at higher ploidy levels [9,[19][20][21][22][32][33][34][35]. While the number of pollen openings has been reported to vary in some polyploidy angiosperms [36], in this study, the number of pores and the furrows of Classopollis remains constant.…”

“…Unreduced gametes have been observed in many modern polyploid mutants and can be generated by a variety of cytological mechanisms [9,10,32]. In plants where unreduced gametes were found, unreduced pollen is formed by anomalies occurring during meiotic cell division.…”

New insights into cytomixis: specific cellular features and prevalence in higher plants

“…The most important mechanism leading to polyploid plants is formation of the gametes with a diploid chromosome number (Bretagnolle and Thompson 1995). Diploid gametes can be produced in several ways, and cytomixis might well be one of them (Falistocco et al 1995;Ghaffari 2006;Lattoo et al 2006).…”

Cytomorphological investigations in Oxyria digyna Hill. from the Kashmir Himalaya, India

“…It has been shown that 2n gametes in Brachiaria brizantha are produced from total absence either of the first or the second cytokinesis [33]. Further, influence of seasonal and environmental factors such as high and low temperature on 2n gamete formation have been shown [31]. Besides, cytomixis might have produced un-reduced pollen grains in this species.…”

“…Un-reduced (2n) pollen grains. The trend to form 2n gametes in plants is highly variable and it varies among individuals within a single taxonomic group or even among flowers of an individual plant [31]. The production of unreduced gametes is heritable, governed by many genes and increases with increasing environmental stress, eg.…”