humpback1 (hmp1) was identified in an EMS pollen mutagenesis M2 screen in our 1993 San Jose nursery (see Harper et al., MNL 69:22, 1995). The phenotype is inherited as a recessive trait and is characterized by proliferation of sheath just beneath the auricle resulting in a bulged sheath. The phenotype is extremely localized to the distal-most part of the sheath just preceding the auricle and ligule and is more apparent on leaves above the ear node. The auricle and ligule are not affected. The sheath phenotype is often more pronounced on either side of the midrib but usually extends from margin to margin. Husk leaves also show tissue proliferation, however the phenotype is localized to the base of the husk leaves and not at the tip, the location of the blade sheath boundary in husks. Phenotypic expression and penetrance are better in a W23 background and poor in both B73 and Mo17 after two generations of introgression. hmp1 was included in our 1994 B-A translocation mapping block and screened for phenotypes in our 1995 nursery. Two independent families from crosses of heterozygous hmp1 by TB-1Sb hyperploid heterozygotes (TB-1Sb/vp5) showed hmp1 phenotype. hmp1 phenotype was not observed in any other TB crosses. We are currently mapping with RFLP and visible markers to further define the location of hmp1 on chromosome 1S.
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