Apomixia autônoma e pseudogâmica em angiospermas : distribuição e relação com o tecido de reserva da semente

Abstract

Apomixis consists of asexual reproduction through seeds. Different mechanisms lead to the autonomous embryo development in apomictics. However, pollination may be necessary (pseudogamy) due to the need for fertilization of the polar nuclei to form the endosperm. In autonomous apomixis, plants are completely independent of pollination for seed formation, and the endosperm is generally produced autonomously. The lack of consolidated data concerning autonomous and pseudogamous apomixis within the realm of angiosperms poses a challenge in elucidating the distribution patterns, prevalence rates, and interrelationships of these states within the taxonomic group. This knowledge gap hinders comprehensive insights into the diverse mechanisms of apomixis and the various modalities of endosperm development. This study aimed to clarify how autonomous and pseudogamous apomixis are distributed among angiosperms and elucidate the type(s) of apomixis (sporophytic, diplospory and/or apospory) recorded in these taxa. Additionally, potential correlation between autonomous and pseudogamous apomixis and the type of seed regarding the permanence of the endosperm at maturity were assessed. Autonomous apomixis is exhibited in at least 111 genera belonging to 47 families. Pseudogamy was found in 79 genera from 23 families. The two conditions cooccur in 19 genera/18 families. Autonomous and pseudogamous apomicts are diffusely distributed among angiosperms, but pseudogamy is not recorded in magnoliids. Asteraceae, Melastomataceae and Orchidaceae have the largest number of genera with autonomous apomixis, while Poaceae and Rosaceae have the largest number of pseudogamous ones. Among autonomous apomictics, sporophytic apomixis, followed by apospory, are the most frequent types of apomixis. Among the pseudogamous, apospory and sporophytic apomixis stand out. More than 75% of genera where autonomous apomixis is recorded exhibit mature nonendospermic seeds. On the other hand, more than 68% of genera with pseudogamy have endospermic seeds. These conditions – autonomous apomixis and non-endospermic seeds, and pseudogamy and endospermic seeds, are correlated. This study highlights that autonomous apomixis has been underestimated in the literature, being much more frequent than previously indicated. The three types of apomixis are recorded in genera with autonomous apomixis and in those with pseudogamy. Finally, characteristics of seed development exhibited by certain groups can be understood as pre-adaptations to autonomous apomixis. These include an ephemeral or non-functional endosperm and the use of ovule tissues in embryonic growth, which must have facilitated the evolution of this condition in some families.