Embryo sac and fertility analyses in a BIII synthetic Paspalum almum hybrid

SCHNEIDER, JUAN SEBASTIAN; ESCOBAR, LUCAS MATEO; DAVIÑA, JULIO RUBEN; HOJSGAARD, DIEGO; HONFI, ANA ISABEL

Rosario

Congreso; IV international congress on Apomixis; 2023

Paspalum almum Chase is a native forage species from northeastern Argentina. Only two cytotypes are known grown in nature, diploids (2n = 2x = 12) and tetraploids (2n = 4x = 24). Diploids and tetraploids have different reproductive modes, diploid plants are sexual self-sterile and tetraploid are facultative apomictic self-fertile. A neo-hexaploid P. almum hybrid plant was recovered from a tetraploid × tetraploid cross. Here we evaluate the embryo sac anatomy in ovules and fertility in this newly formed hexaploid aiming at assessing its reproductive mode and possible incorporation into current breeding schemes. Three inflorescences were collected at anthesis and 93 cleared pistils were analyzed. Embryo sacs were classified in meiotic Polygonum-type and aposporous Paspalum-type. Seed set was determined from 10 inflorescences under open-pollination and Fertility Index (FI) was calculated as Number of seeds per inflorescence / Number of spikelets per inflorescence. We found three categories of ovules. Ovules carrying only meiotic embryo sacs (0.54), ovules carrying one meiotic + 1 or more aposporic embryo sacs (0.13 in total, grouped in 0.11 with 1 aposporic embryo sac, 0.01 with 2 aposporic embryo sac; and 0.01 with 3 aposporic embryo sac) and ovules with immature embryo sacs, with clear signs of abortion or no embryo sacs (0.33). FI was 0.03 (82 seeds were obtained from 1062 spikelets). The data indicates that, despite the high proportion of non-viable ovules, much of those viable ones carrying functional embryo sacs failed to develop into a seed. The low fertility of the hexaploid plant is likely responding to the genomic stress caused by the ploidy shift and gene dosage imbalances. Upcoming studies on pollen-pistil compatibility, self-pollinated seed-set, and meiotic and genetic analyses will help us to unravel the cytological and molecular basis of this sterility.