Procedures to improve Stock 6
- Chang, MT; Coe, E

Stock 6 was named by Coe in his genetic collection. Ed recognized its high haploid induction rate in 1952. He then converted Stock 6 in 1960 to carry homozygous A B Pl C R-nj anthocyanin genes, expressing purple plant color, purple plumule and purple seed crown for easy identification of haploid seeds. The induction rate of Stock 6 is about 2% to 3% with poor agronomic traits such as poor stalk, poor roots, easy to lodge and ear rot. These poor traits were improved by Chang with advice from Coe according to the following procedures, and the rate of haploid induction was increased by selection.

A green plant, yellow seed unfixed material (AA cc rr bb plpl) that had shown it was producing haploids in the field was used as female and crossed by purple plant, purple seed crown and purple plumule Stock 6 (AA CC R-njR-nj BB PlPl) pollen. 20 F1 seeds (A/a C/c R-nj/r B/b Pl/pl) were planted and selfed for F2 seeds.

The F2 seeds segregated nine colored crown and plumule seeds vs. seven colorless seeds. Selected and planted 200 F2 seeds of the most dark-colored crown and plumule to enhance the probability of homozygous AA CC R-njR-nj plants. Selfed F2 plants with dark purple leaves. Ears should either segregate purple crown and colorless seeds or all homozygous colored seeds. Selected the homozygous ears that showed seed color fixation (AA CC R-njR-nj).

Planted 50 F3 seeds from each ear with seed color fixed. Identified the most dark purple leaf, sheath, and tassel plants and selfed. These plants were supposed to be homozygous for AA BB PlPl genes. Planted 20 ear-to-row F4 seeds from each selfed F3 ear. Selected the rows showing all purple plants to confirm plant color fixation, and selfed all ears.

The genetic make-up of F5 seeds was fixed for AA CC R-njRnj BB PlPl. The phenotypic expression was all purple plants and purple crown and plumule. Planted 200 F5 seeds and also 500 hybrid seeds as tester.

Selfed each F5 plant and carried pollen to cross onto two hybrid ears. Harvested all selfed F5 ears with ear number -1, -2, -3 and so forth. Also harvested all crossed ears with pollen source X-1, X-2, X-3 and so forth.

Screened all crossed ears for haploid seeds production. For example, there should be two ears crossed by -1 F5 plants or X-1. If one ear showed 6% haploids and the other ear showed 4% haploids, then the -1 F5 plant had a 5% induction rate. If one ear showed 9% and the other ear 4%, the induction rate was too far apart, so the lower 4% was assigned to define the F5 plant induction rate. Screened all the ears and sorted induction rate from low to high. A range of induction rate from almost 0% to 8% was obtained.

Selected the highest rate F5 ears and planted 20 F6 seeds earto- row from 10 ears to form a 200-seed population. Also planted 500 hybrid seeds as tester, using the above procedures again to determine induction rate of each plant. The induction rate was increased in the range of 1% to 9% in response to selection. Selected the highest induction rate F6 ears and planted 20 F7 seeds ear-to-row from 10 ears to form a 200-seed population. Also planted 500 hybrid seeds as tester. Repeated the above procedures. Also planted 200 F7 seeds with highest induction rate, selfed the haploid plants to form a new pure Stock 6 line with higher induction rate and better phenotypic traits.

Repeated the above procedures for a couple of more generations. The F10 seeds from higher induction F9 ears were sibbed to retain plant vigor and progeny were sibbed again and again. From the progenies, several high induction lines were produced to form new Stock 6 lines. The average haploid induction rate is 6% to 8%. Seeds of this improved Stock 6 are being sent to the Maize Genetics Cooperation Stock Center.