Rootworm Resistance in F1 Tripsacum X Zea diploperennis
A bioassay for resistance to western corn rootworm Diabrotica virgifera LeConte in F1 progeny of a cross between Tripsacum dactyloides (L.) L. and Zea diploperennis Iltis, Doebley & Guzman was conducted to determine if rootworm resistance is inherited in offspring of the original hybrid plant designated Tripsacorn (MNL 67:39-40). If resistance occurs in the F1, tests are needed to determine if the trait segregates according to Mendelian inheritance.
Seed was germinated on moist filter paper in petri dishes. Twelve seedlings were then planted in potting soil in 2.25 inch square plastic pots and grown indoors under a 33 watt fluorescent grow light. For infestation, 1,000 non-diapausing western corn rootworm (WCR) eggs in soil were shipped from French Agricultural Research, Inc., Lamberton, MN, to Seeds for the Future, Durham, NC, under USDA permit no. 922762. At 42 days old, when plants had 2 to 4 leaves and were approximately 4 inches tall, 25 newly hatched WCR first instar larvae were transferred to pots using a small paintbrush. Four days later 25 additional larvae were transferred to the twelve pots, giving an infestation total of 50 larvae per pot. Ten days after final infestation, individual plants were immersed in a container of water and roots gently washed for subsequent examination.
There was a range of variation from plants with roots that had been severely chewed and died, to healthy plants with vigorous growth whose roots exhibited almost no larval feeding. The results of this preliminary bioassay to screen for rootworm resistance in progeny indicate a gene for rootworm resistance that has a dosage effect is inherited from Tripsacum. Presumably, plants with severely chewed roots that died were homozygous susceptible; plants intermediate in root feeding and growth were heterozygous, and healthy plants with minimal root feeding were homozygous resistant. The ability to use homozygous resistant plants for crossing would greatly facilitate a breeding program to introduce rootworm resistance into maize via the Tripsacorn bridge species. An experiment to determine if results of laboratory bioassays can be replicated in greenhouse infestations is underway.
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