A mutation, disturbing the orientation of germ in relation to cob, i.e., the embryo facing the stalk end of the ear as opposed to the tip end in normal situation, has been recovered in the course of a mutagenesis study. Progeny raised from these reversed embryo cases produced ears with all the germs facing the base of the ear or having both normal and abnormal germ orientation with variable frequencies. The mutant showed poorer seed viability and weaker plant vigor as compared to normal sibs. A similar mutation disturbing the germ orientation was reported earlier by Brieger (MNL 20:55, 1948) and Joachim (MNL 29:53, 1955; 30:84-85, 1956). Joachim designated this trait as "reversed germ" and explained their occurrence as due to the development of both the florets in an earshoot resulting in two kernels facing each other. She observed that in Country Gentleman sweet corn 50% of kernels were reversed in the part of the ear where no distinct rows could be detected.
In our case, germ reversal was not always associated with crowding of kernels and irregularities of rows as reversal was noticed in many straight-rowed ears. The reversed position of the germ in relation to the ear shoot seems to be due to modified orientation of the ovule. Differential growth of integuments and nucellus at the posterior side of the ovule during normal development pushes the ovule to one side by 90o. If the ovule is pushed on the other side to the same extent, the germ would show 180o turning in respect to the normal position in mature cobs. This probably is the cause for origin of reversal in the reversed germ cases. Several kernels were found in few ears where this reversal was incomplete, showing part of the embryo on top of the kernels. The position of the embryo in relation to the silk scar was found to be unchanged. Hence, other possibilities like the twisting of the pedicel to bring about 180o turning of the germ to face downward or interchange of relative position of embryo and endosperm initials in the embryo sac, i.e., turning of embryo sac within the ovule, seem to be less likely.
It is a maternal plant character and in crosses with normal orientation, expression of this trait depends on the female parent. When reversed embryo was used as female, the ears in F1 plants showed this trait but were all normal when used as male. In F2, a 3:1 segregation for normal and abnormal ears was obtained. Penetrance of this trait seemed to be complete but the ears exhibiting this characteristic showed variable expressivity of this gene. In the testcrosses an expected 1:1 segregation was realised depending on the choice of the female parent. The percentage of reversed germ kernels in the ear could be progressively increased through selection, suggesting the presence of modifiers. Among ears showing this trait in F2 74% had less than 10% reversed germ kernels and only 8% had more than half of the kernels in the ear with reversed germ orientation.
We propose a three letter gene symbol, rgo, for this reversed germ orientation trait.
J. K. S. Sachan and K. R. Sarkar
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