Apomixis and sexuality in diploid and tetraploid accessions of Brachiaria decumbens

Abstract

Meiotic and aposporous embryo sacs and the initial steps of parthenogenetic embryogenesis and endosperm formation were investigated in diploid and tetraploid accessions of Brachiaria decumbens in two environments, differing mainly in day length: early summer and late autumn. Both diploid and tetraploid accessions were facultative apomicts. Di(ha)ploids showed a much lower level of apomixis (10% to15%) than tetraploids (80% to 95%). No obligate sexual diploids were found; thus, their occurrence in natural populations is obscure. It is suggested that reproduction in B. decumbens, as in other agamic complexes of the Paniceae tribe, in general, approximates a diploid-tetraploid-(di)haploid reproductive cycle which does not involve triploids. The dihaploids were fertile and survived in nature. Development of the reproductive structures depended on the environment. In autumn, in contrast to early summer, many meiotic and aposporous embryo sacs degenerated during development, leading to a significant reduction in the proportion of parthenogenetic embryos. Whether this effect can be attributed to day length or simply to age remains to be investigated. The ratio of aposporous to sexual embryo sacs was relatively stable over the two seasons.& k w d : Key words Apomixis · Sexuality · Day length · Embryology · Brachiaria · Agamic complexes& b d y :

Levels of apomictic and amphimictic seed formation in a natural population ofCoprosma robusta(Rubiaceae) in Riccarton Bush, Christchurch, New Zealand

“…It has been suggested that, for many species, only diploid or polyploid gametes may carry the alleles for apomixis and that haploid gametes are selected against or have developmental abnormalities (Nogler 1984;Naumova et al 2001). In some cases, however, diploid apomicts have been recorded from the field (Jankun & Kovanda 1988;Naumova et al 1999Naumova et al , 2001) and haploid transfer of apomixis has been noted (Bicknell et al 2000), indicating that this effect may be associated with the genomic organisation of individual taxa, rather than a feature of apomixis per se. Given the close association of apomixis and polyploidy, it is perhaps most likely that in Coprosma the functional diploid chromosome number of 2n = 44 actually represents a tetraploid complement with a basic number of x = 11.…”

Picking up the Ball at the K/Pg Boundary: The Distribution of Ancient Polyploidies in the Plant Phylogenetic Tree as a Spandrel of Asexuality with Occasional Sex

“…One of them is called the "diploid-tetraploid-diploid cycle." Paspalum is one such genus where a breeding population mostly consists of facultative apomictic tetraploids, sexual diploids, triploids, and aneuploids (see Naumova et al [1999] for a relatively recent reference). Thus, there are populations that could, in theory, help establish a polyploid "event."…”

Reproductive development in apomictic populations of Arabis holboellii (Brassicaceae)

“…Ovule and seed development For the study of ovule and seed development, whole ovaries and siliques were fixed in either FAA or Carnoy's solution and cleared as in Naumova et al (1999). The ovules and seeds from individual ovaries and siliques, respectively, were mounted on separate slides for further investigation.…”

“…Ploidy levels of leaf cells were determined using flow cytometry. Suspensions of intact leaf nuclei were prepared (Arumuganathan and Earle 1991;Naumova et al 1999) and analyzed using a Beckman-Coulter EPICS XL-MCL flow cytometer (Miami, Fla.) equipped with an argon ion laser at 488 nm. The ploidy of embryo and endosperm nuclei was determined in single mature seeds using a Partec Flow Cytometer PA (Münster, Germany) and the Flow Cytometric Seed Screen (FCSS; Matzk et al 2000Matzk et al , 2001.…”