Genetics of Apomictic Embryo Sac Formation and other Morphometric Variables of Ovule Development in Sorghum.

Low frequency apomictic (aposporous) embryo sac (AES) formation in Sorghum bicolor L. Moench has been documented in several genotypes.  However, AES functionality in sorghum (parthenogenesis followed by viable seed formation) has not been definitively documented.  In searching for functional apomixis in sorghum, we screened 53 divergent accessions.  Frequency data (percent of ovules expressing a trait) was obtained by differential interference microscopy for the following traits:  aposporous initials (enlarged nucellar cells), large stack cells (enlarged chalazal cells immediately chalazal to the megaspore mother cell), AES (vacuolate AIs with 1-8 nuclei), persistent tetrads (embryo sac formation initiating from other than the chalazal megaspore), twin MMC/meiocytes, and ovule degeneration.  Ovule morphometric (area and angle) measurements were also taken at i) the dyad to early tetrad stage of meiosis, ii) the 2-nucleate embryo sac (ES2) stage and the early ES8 stage.  Few lines produced AES, and frequency AES seldom exceeded 4%.  The highest frequency AES formation observed was 13%, which occurred in a natural tetraploid.  Several lines with high AES formation were screened for parthenogenesis.  Evidence for parthenogenesis (< 0.5% of ovules) was observed in one line.  To understand the genetics of AES formation and other traits, we produced a mapping population consisting of 302 F₂.  This and a second mapping population (IS3620C RILs from Bob Klein) were phenotyped as described above, and all traits segregated.  Frequency of ovules containing AI or AES per F₂ ranged from 0 to 23 % with the distributions being highly skewed toward 0 %.  Many F₂ that underwent an early meiosis and an early sexual embryo sac formation, relative to integument development, produced significantly more AES.  This effect was collaborated by the larger study involving 53 genotypes.  Details of QTL mapping of each trait will be presented.