Sporophytic apomixis in polyploidAnemopaegmaspecies (Bignoniaceae) from central Brazil

Abstract

Apomixis and polyploidy have been important in the evolution of the angiosperms, and sporophytic apomixis has been associated with polyembryony and polyploidy in tropical floras. We studied the occurrence of polyembryony in populations of tetraploid Anemopaegma acutifolium, A. arvense and A. glaucum from the Brazilian cerrados, and histological features of sexual and apomictic processes were investigated in A. acutifolium. All populations and species were polyembryonic (68.9-98.4% of seeds). Normal double fertilization occurred in most ovules, with exceptions being that 3% of ovules were penetrated but not fertilized and in 4% of ovules both synergids were penetrated. The penetration of both synergids suggests a continuous attraction of pollen tubes and polyspermy. Adventitious embryo precursor cells (AEPs) arose from nucellar and integumental cells of the ovule in pollinated and unpollinated A. acutifolium, indicating sporophytic apomixis. However, further embryo and endosperm development required pollination and fertilization. This pseudogamy also allows concurrent sexual embryo development. Similar polyembryony rates and polyploidy indicated that A. arvense and A. glaucum are also apomictic, forming an agamic complex similar to that observed for some species of confamilial, but not closely related Handroanthus. The co-occurrence of apomixis and polyploidy in different groups of Bignoniaceae indicates homoplasious origin of these agamic complexes.

Self-sterility in the hexaploid Handroanthus serratifolius (Bignoniaceae), the national flower of Brazil

“…This result is new to Bignoniaceae, because the neopolyploid species of Anemopaegma and Handroanthus studied to date are clearly self-compatible (Bittencourt & Semir 2005;Bittencourt & Moraes 2010;Firetti-Leggieri et al 2013;Sampaio et al 2013a). As these neopolyploid and self-compatible species also present polyembryony caused by sporophytic apomixis (Costa et al 2004;Bittencourt & Moraes 2010;Sampaio et al 2013b), it is possible that the self-fertility observed was trigged by the development of adventive embryos, as previously hypothesized (Oliveira et al 1992;Bittencourt & Semir 2005;Bittencourt & Moraes 2010;Sampaio et al 2013a). Our data suggests that neopolyploidy does not seem to be the main factor causing self-fertility in Handroanthus species.…”

“…If the studied plants were sporophytic apomicts, high proportions of polyembryonic seeds (> 20%) were expected. However, a low proportion of polyembryonic seeds (< 5%) can be attributed to some exceptions in the sexual process or to other types of apomixis (Mendes-Rodrigues et al 2012, Firetti-Leggieri et al 2013Sampaio et al 2013b). One investigation of the proportion of polyembryonic seeds in a population of Handroanthus serratifolius showed little variation among individuals (Mendes-Rodrigues et al 2012).…”

“…Neopolyploid species (i.e., polyploids whose closely related diploid species are not extinct) have been found in the genera Anemopaegma (Firetti-Leggieri et al 2011, Sampaio et al 2013b, Dolichandra (Piazzano 1998), Handroanthus (Piazzano 1998;Sampaio 2010) and Pyrostegia (Goldblatt & Gentry 1979;Sampaio 2010), with chromosome numbers in somatic cells of 2n=60, 2n=80 and 2n=120, respectively. Studies of the breeding systems of the neopolyploid species of Anemopaegma and Handroanthus have shown self-fertility and polyembryony (Bittencourt & Semir 2005;Bittencourt & Moraes 2010;Mendes-Rodrigues et al 2012;Firetti-Leggieri et al 2013;Sampaio et al 2013a).…”

“…ex DC) Mattos (Bittencourt & Moraes 2010) and Anemopaegma acutifolium DC. (Sampaio et al 2013b), indicating the occurrence of sporophytic apomixis. On the basis of those findings, high frequencies of polyembryonic seeds have been used as evidence of sporophytic apomixis in Bignoniaceae (Mendes-Rodrigues et al 2012, Firetti-Leggieri et al 2013Sampaio et al 2013b).…”

“…The LSI found in most species of Bignoniaceae allows pollen tube growth, ovule penetration, fertilization and endosperm development prior to self-pollinated pistil abscission (Bittencourt et al 2003;Bittencourt & Semir 2005;Gandolphi & Bittencourt 2010). In apomictic species, adventitious embryos develop irrespective of pollination but are able to reach maturity only when the endosperm is available (Bittencourt & Moraes 2010;Sampaio et al 2013b). Therefore, the presence of adventitious embryos in self-fertilized ovules with endosperm development, which is common in LSI species, could trigger the fruit-set of self-pollinated pistils (Oliveira et al 1992;Bittencourt & Moraes 2010;Sampaio et al 2013a).…”