Hybrid brinjal seed production with a male-sterile line
An F1 hybrid brinjal (Solanum melongena) seed programme has 1,200 plants. Brinjal flowers are hermaphrodite and normally self-pollinate, and anthers dehisce in the early morning. The breeder has a line carrying a spontaneous male-sterile mutation: its flowers have normal, receptive stigmas and set normal fruit when hand-pollinated, but its anthers are shrivelled and release no viable pollen. The male parent line is a high-yielding inbred with normal pollen. The field is surrounded by other farmers' brinjal fields, and several insect species visit the flowers. The breeder plans to interplant the two lines, collect seed from the male-sterile plants only, and sell it as F1 hybrid seed. The two lines flower over 10 weeks, and the male-sterile line is uniform. A field assistant suggests that because the male-sterile line cannot self-pollinate, bags and isolation distance are unnecessary, and that pollen can be collected from the male parent once and stored in water for spraying later.
1. The assistant claims bags and isolation distance are unnecessary. Evaluate this claim and describe the minimum protocol that would make the harvested seed acceptable as F1 hybrid seed.
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The claim confuses absence of selfing with absence of unwanted pollination. Male sterility only stops the seed parent from using its own pollen; the stigma is still receptive, and insects can bring pollen from nearby brinjal fields. Any such pollen produces off-type seed, and because brinjal is mainly self-pollinated, the contaminating pollen may come from plants that are not the chosen male parent. The harvested seed would therefore be a mixture, not F1. The seed parent's sterility must also be verified: if fertility is temperature-sensitive, a few flowers may shed pollen and set selfed fruit.
Minimum protocol:
- Confirm that every plant in the seed-parent block is male sterile and rogue out any fertile plant.
- Bag unopened flowers of the seed parent with butter-paper bags before anthesis, or grow the block in an insect-proof net house.
- Collect fresh pollen from the male parent and hand-pollinate.
- Re-bag and label each crossed flower.
- Harvest fruit only from the male-sterile plants.
Isolation distance alone is less reliable in a dense brinjal area; for a small programme, bagging is the safer control. Seed from the male-parent block is not harvested as hybrid seed because it is selfed.
2. Outline a workable pollination schedule, including emasculation, pollen collection and stigma receptivity, and explain why emasculation is omitted on the male-sterile line but would be required on a fertile female parent.
Read the worked answer
Anthesis in brinjal is in the early morning. On a fertile female parent, emasculate the day before anthesis: open the corolla carefully and remove the anthers with forceps before they dehisce, leaving the stigma undamaged; then bag the flower. On the male-sterile line, no anthers need removing because no viable pollen is produced, but the flower must still be bagged before anthesis to exclude insect pollen.
Collect male-parent flowers at anthesis, tap the anthers over a clean Petri dish, and use the pollen the same morning. Pollen is desiccation-sensitive and short-lived; storing it in water is wrong because water causes osmotic bursting and germination is lost. If storage is unavoidable, keep it dry and cool (e.g., 4°C) for a day or two and test viability. Pollinate when the stigma is receptive, generally from one day before to the morning of anthesis, by dusting pollen on the stigma. Re-bag, tag with date and parent identities, and harvest mature fruit. Ferment and wash the seed, then dry. Keeping a dated log is essential because fruit from different crosses must not be bulked.
3. Four percent of the male-sterile block is found to have a few viable pollen grains and some selfed fruit. What are the consequences, and what should be done?
Read the worked answer
Those fruits are a mixture of hybrid and selfed seed. Selfed seed from a male-sterile plant will produce progeny that are also male sterile (if the sterility is genetic) or may revert to fertility (if it is temperature-sensitive), but in either case they are not the intended F1. In the farmer's field, non-hybrid plants reduce uniformity and yield, and male-sterile plants may set little fruit unless pollinated.
Action: mark and remove every plant in the seed-parent block that shows anther dehiscence or fertile pollen; do not harvest fruit from those plants. Harvest only from plants that were confirmed sterile and bagged. Check the stability of the sterility at the temperatures expected during seed production; if it breaks down, use a cytoplasmic male-sterility system with a maintainer line instead of a spontaneous mutation, or shift seed production to a cooler season. Keep a sample of the harvested seed for a grow-out test: sow a sample and score for uniformity and male sterility before the lot is sold. If contamination is already high, discard the lot rather than sell it as F1.