Penetrance and expressivity of twin ears

Twin ears were first observed in an S5 progeny of BSAA o2 and an S2 progeny of BS10(FR)C2 (MNL 58:21). Progenies were continued in the breeding nurseries by self-pollination and ears were selected from plants with the twin-ear expression. In 1985 and 1986, data were collected from 25-plant nursery rows to obtain estimates of penetrance and expressivity of 65 progenies derived from the original two ears. The original twin ears were at the top-ear node, but twin-ear expression also was observed at a lower frequency at the 2nd and 3rd nodes. In addition to recording the relative penetrance in 1986, ears were harvested from all pollinated plants within the nursery rows to determine the relative expressivity of twin-ear expression. Data collected in 1985 and 1986 are summarized here along with six selected progenies to illustrate the differences between years for penetrance and for expressivity in 1986.
 
  Expressivity
Penetrance(%)
Seed

Number

Seed

Weight

1985
1986
Twin
Single
Twin
Single
Average (all progenies)
83
79
401
234
100
55
Range (all progenies)
64-100
56-100
384
738
78
118
Selected progenies
1673-3031A 
100
86
415
---
104
---
1678-3042A 
76
81
578
---
100
---
1686-3062B 
76
100
184
---
51
---
1691-3065A 
100
95
363
---
96
---
1694-3067B 
93
100
332
---
82
---
1698-3074A 
95
95
359
---
103
---
Average
83
79
379
---
89
---

The frequency of penetrance at the top-ear node averaged 83% in 1985 and 79% in 1986. Some progenies had 100% expression for twin ears, but none expressed 100% twin ears in both years; 1691-3065A had 100% expression in 1985 and 95% in 1986, whereas 1694-3067B had 93% expression in 1985 and 100% in 1986. Twin-ear expression at the second node was less than the top node in both years with an average twin-ear expression of 32% in 1985 and 54% in 1986. The frequency of twin ears at the second node was 22% greater in 1986 vs. a 4% lower frequency at the top node.

Data were collected for each of the twin ears for seed number and weight and for the one-ear segregates to estimate expressivity. Each pair of twin ears was visually designated as largest and smallest of the two ears. Average numbers of seeds for each ear of the pair were 219 and 182, or 401 seeds for 91 pairs of twin ears; the 27 one-eared segregants averaged 234 seeds per ear. The twin-eared plants had 71.4% more seeds than the one-eared plants. Average total seed weight for the twin-eared plants (100.1g) was 81.8% greater than for the one-eared plants (55.4g). If total seed weight is divided by total seed number to estimate the average seed size, average weight of each seed was 0.25g for the twin-ear plants vs. 0.24g for the one-ear plants. The twin-ear plants, therefore, had greater numbers of seeds that were similar in size to the seeds of one-eared plants.

Testcrosses of 41 twin-eared plants, used as females, were produced in 1986 with lines (B77, B79, B88, B90, B91, and Mo17) that did not exhibit twin ears. Only three twin-ear plants were observed in the testcrosses and all occurred in testcrosses with Mo17. Crosses of twin-eared progenies with other twin-eared progenies, however, tend to increase the frequency of twin ears. Twin ear seems to be a threshold trait that may be difficult to stabilize. Although some progenies have a high frequency of expression in different years, it may be difficult to transfer to other lines. Greatest utility of twin-ear expression may be for genetic studies in which multiple pollinations on a single plant are desired.

Arnel R. Hallauer


Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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