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a Kemptville College, University of Guelph, Kemptville, ON, K0G 1J0 Canada
b Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Research Centre, Ottawa, ON, K1A 0C6 Canada
c Dep. of Plant Science, McGill University, 21111 Lakeshore, Ste-Anne-de-Bellevue, QC, H9X 3V9 Canada
* Corresponding author (diane.mather{at}mcgill.ca)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In hulled oat, it is common to find aberrant plants that resemble the parental variety except for the presence of a large geniculate (bent) awn on the lemma of each floret and a scar, also called a "suckermouth" (a sunken, round or oval disarticulating zone bordered by pubescence) at the base of the lemma of each floret. At maturity, the suckermouth has completed its development and facilitates grain shattering (Huskins, 1946). These aberrant A. sativa plants are called "fatuoids" because the geniculate awn and the suckermouth are characteristic of the hexaploid wild oat, Avena fatua L. Because of their tendency to shatter and the fact that their heavy lemma awns detract from the appearance of the grain, fatuoid lines of hulled oat cannot be used as cultivars.
Fatuoids also occur in naked-grained types of A. sativa. The fatuoid naked oat materials with which we have worked have twisted geniculate awns on the lemmas of their primary and secondary florets, but they do not exhibit the suckermouth or shatter (Hoekstra et al., 2001). The heavy lemma awns do not affect the appearance of the grain because, in naked oat, groats thresh free of the lemma and its awns. Thus, it is possible to use fatuoid naked oat lines as cultivars, and one of us (VDB) has released such cultivars (AC Baton and AC Ernie, registration numbers 3963 and 4559, respectively, Variety Registration Office, Canadian Food Inspection Agency). We have repeatedly observed a low incidence of hulled grain in some fatuoid naked oat lines (Hoekstra, 1998, and unpublished data).
Most published reports on the naked-grained character in oat have proposed that it is governed by a single gene. Norton (1907), Gaines (1917), Zinn and Surface (1917), Caporn (1918), Love and McRostie (1919), Chou (1932), Clamot (1969), Boland and Lawes (1973), and Kibite and Taylor (1994) observed F2 ratios of approximately 3 naked-grained or mosaic plants:1 hulled plant. Mosaic plants produced both naked and hulled grain. The proportions of naked-grained plants and mosaic plants varied from cross to cross, but when totaled, the naked and mosaic classes consistently comprised about 75% of the F2 plants. The 3:1 ratio suggests that the naked versus hulled character is monogenic. The hulled allele acts as a typical recessive allele since the hulled character breeds true in the F3 and subsequent generations, while the progeny of naked-grained plants are not all naked grained. Gaines (1917) and Love and McRostie (1919) observed both naked-grained and mosaic plants within F3 plants derived from some F2 plants that were completely naked grained. Love and McRostie (1919) assumed that some hulled F2 plants had been incorrectly classified as naked (some hulled grains dehulled at threshing), or that other factors affected the expression of the naked condition. In some studies (Love and McRostie, 1919; Chou, 1932; Kibite and Taylor, 1994), 1:2:1 ratios (i.e., 1 naked-grained plant: 2 mosaic plants: 1 hulled plant) were observed. These are consistent with the segregation of a single codominant gene.
Multigene models have also been proposed for the naked character on the basis of observations of F2 progeny that deviated from a ratio of 3 naked or mosaic plants:1 hulled plant. Both Moule (1972) and Jenkins and Hanson (1976) proposed models including a locus at which the dominant allele (N-1) causes the production of naked grains and the recessive form (n-1) causes the hulled phenotype. Moule (1972) also proposed the existence of two incompletely dominant modifying genes and Jenkins and Hanson (1976) suggested a third modifying gene. The Jenkins and Hanson model assumes complex epistatic relationships and results in an F2 segregation ratio of 35 naked plus mosaic plants:29 hulled plants. In an attempt to confirm their four-gene model, Jenkins and Hanson (1976) observed and classified F2:3 families. The hulled phenotype was more frequent in the F3 generation than it had been in the F2, and unexpectedly, some fully naked F2 plants produced fully hulled F2:3 families. The four-gene model could not be confirmed because of wide discrepancies between expected and observed proportions. Jenkins and Hanson (1976) attributed the increase in the proportion of hulled grain in the F2:3 families compared with F2 plants largely to environmental influences.
Studies of fatuoids in hulled oat have indicated that they originate as mutants that lack the gene or genes responsible for suppression of the fatuoid character (Huskins, 1946; O'Mara, 1961). They may result from the loss (deletion) of a single gene (series-
fatuoids), a complete chromosome (series-ß fatuoids), or chromosomal segments (series-
fatuoids). In crosses between hulled homozygous -fatuoids and hulled homozygous nonfatuoids, the F2 generation has been observed to segregate in a 1:2:1 ratio (i.e., 1 awnless plant:2 plants with one awn per spikelet:1 plant with two or more awns per spikelet) (O'Mara, 1961). This is consistent with control of the fatuoid character by a single codominant gene.
We are not aware of any published reports of genetic studies using fatuoids or A. fatua as parents with naked-grained A. sativa, but Zhegalov (1924) studied a cross between A. sterilis subsp. ludoviciana (Durieu) M. Gillet & Magne and naked-grained A. sativa. The A. sterilis parent and hulled F2 plants with two or more awns per spikelet exhibited the suckermouth phenotype and were prone to shattering. None of the naked-grained F2 plants had suckermouth structures at the base of florets, and their seed did not shatter. This apparent masking of the suckermouth by the presence of the naked condition is similar to what we have observed in fatuoid naked oat cultivars and breeding material.
Here, we examined the inheritance and expression of the naked-grained and fatuoid characters in crosses involving two fatuoid naked oat lines.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The fatuoid naked oat line NO753-2 was crossed with Marion and with Terra. The fatuoid naked oat line NO753-2A was crossed with Marion, with Terra and with Tibor. In each case, the fatuoid naked oat line was used as the female parent. Reciprocal crosses were not made because previous research has shown that neither the naked character (Jenkins and Hanson, 1976) nor the fatuoid character (Huskins, 1946) is maternally inherited. One to three F1 plants were grown for each cross, providing one F2 population each of NO753-2/Terra and NO753-2A/Tibor, two replicate F2 populations for NO753-2A/Terra and three replicate F2 populations for each of NO753-2/Marion and NO753-2A/Marion.
F2 Evaluation
F2 seeds were sown in the second week of May 1989 at Winchester, ON, with all F2 seeds of any one cross sown on the same day. Plants were grown at 10-cm spacing within rows 45 cm apart. Plants were harvested individually. In the crosses involving Marion, some F2 plants (fatuoids with hulled grain) were expected to shatter, so all plants from these crosses were cut and stored in paper bags. In the remaining crosses, the F2 plants were not expected to shatter, so they were pulled and stored as intact plants.
One panicle from each plant was examined to determine the maximum number of geniculate lemma awns per spikelet. Panicles without any geniculate lemma awns were classified as awnless. Panicles with geniculate lemma awns typically exhibited some within-panicle variation for the number of awns per spikelet, but were classified as having up to one, two, three, or four awns per spikelet according to the highest number of geniculate awns observed on any spikelet within the panicle. For the crosses involving Marion, one panicle from each F2 plant was photocopied immediately after harvesting. This was done to permit determination of the number of awns per spikelet before shattering occurred. In the remaining crosses, where the F2 plants were not expected to shatter, the numbers of awns per spikelet were determined directly from the panicles.
Individual plants were gently threshed by hand with a soft cloth. Grains were classified as naked (having no hull tissue adhering to the groat) or hulled. Hulled grains were further classified as intermediate (having hull tissue loosely adhering to the groat) or covered (having hull tissue tightly adhering to the groat). In the NO753-2/Marion and NO753-2A/Marion crosses, each plant was then classified as naked (with the proportion of naked grain as high as in the naked-grained parent), hulled (with hulled grains only), or mosaic (with both naked and hulled grains).
F2:3 Family Evaluation
In each of the five crosses, up to 12 individual F2 plants, including five or six with high proportions of naked grain (i.e., the cleanest threshing F2 plants) and five or six with low proportions of naked grain (i.e., the poorest threshing F2 plants), were chosen from each of four categories of plants on the basis of awn number per spikelet (i.e., awnless, one awn per spikelet, two awns per spikelet, three awns per spikelet) to be advanced to the F3 generation. On 29 and 30, May 1990, F2:3 families were sown at Winchester ON, with families nested within crosses in two blocks. On 29 May 1991, all families were sown in a randomized complete block design in three blocks. Each plot consisted of one 6-m row into which 50 seeds (in 1990) or 33 seeds (in 1991) were sown. Those rows were 35 cm apart, with one row of spring-sown winter wheat (Triticum aestivum L.) between them to provide competition against weeds. At maturity one panicle was harvested from each F3 plant. For each of these panicles, the maximum number of awns per spikelet was recorded. In the NO753-2/Terra, NO753-2A/Terra, and NO753-2A/Tibor crosses, each of these panicles was gently threshed with a soft cloth. Grains were classified as naked, intermediate, or covered and the number of grains in each category was recorded for each panicle.
Statistical Analysis
In each of the replicate F2 populations of the NO753-2/Marion and NO753-2A/Marion crosses, the numbers of naked-grained plants, mosaic plants, and hulled plants were compared with a 1:2:1 ratio (1 naked:2 mosaic:1 hulled) by 
2 tests for goodness-of-fit (Steel and Torrie, 1980). Because the plants had been gently hand threshed, it was possible to identify intermediate grains, and these were considered as hulled grains when the plants were classified. In some previous studies, more vigorous threshing was probably used. With vigorous threshing, many or all-intermediate grains would be dehulled and would be classified as naked. This could in turn affect the classification of the plants as naked grained, mosaic, or hulled. To permit comparisons with the results of such studies, two alternative phenotypic ratios were tested, a gentle threshing ratio in which the intermediate grains were classified as hulled grains, and a vigorous threshing ratio in which the intermediate grains were deliberately misclassified as naked grains.
In each of the F2 populations of each of the five crosses, observed phenotypic ratios for awn number were compared to a 1:2:1 ratio (1 awnless plant:2 plants with one awn per spikelet:1 plant with two or more awns per spikelet) and to a 3:1 ratio (3 plants with no awns or one awn per spikelet:1 plant with two or more awns per spikelet) by 2 tests for goodness-of-fit.
Where both characters were segregating and where observed ratios for both characters met expectations for single-gene models, the recombination fraction between a putative gene for naked grain and a putative gene for fatuoidy was estimated by the product-ratio method (Immer and Henderson, 1943).
Within F2:3 families derived from awnless F2 plants and from F2 plants with one awn per spikelet, observed phenotypic ratios for awn number were compared with1:2:1and 3:1 ratios, by 2 tests for goodness-of-fit.
For the crosses involving Terra and Tibor, the ratio of naked to hulled grains (with both intermediate and covered grains considered to be hulled) for F2 plants without awns or with one awn per spikelet were compared to the corresponding ratios for F2 plants with two or more awns per spikelet, by Fisher's exact probability test (Daniel, 1978). This comparison was also conducted within F2:3 families derived from F2 plants with one awn per spikelet.
All statistical tests were conducted at a significance level of 0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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For the number of geniculate awns per spikelet (i.e., the fatuoid character), all of the F2 populations from the crosses involving Marion deviated significantly from the 1:2:1 ratio (i.e., 1 awnless plant:2 plants with one awn per spikelet:1 plant with two or more awns per spikelet), with more than the expected number of awnless plants and less than the expected number of plants with one awn per spikelet (Table 1). With these two phenotypic classes combined, only one population deviated significantly from a 3:1 ratio (i.e., 3 plants with no awns or one awn per spikelet:1 plant with two or more awns per spikelet). That population had less than the expected number of plants with two awns or more per spikelet. When its data were combined with those from the other two replicate populations from the same cross (NO753-2/Marion), there was no significant deviation from the 3:1 ratio. Given that previous studies (Huskins, 1946; Coffman, 1964) had found that variation in the fatuoid character in hulled oat germplasm could be explained by the segregation of one codominant gene, we investigated the possibility that the hulled progeny of the NO753-2/Marion and NO753-2A/Marion crosses would satisfy single-gene models better than the naked-grained progeny of the same crosses. We found no strong evidence to support this idea (data not shown).
For the three replicate F2 populations from NO753-2A/Marion, in which neither character deviated significantly from the 3:1 ratio (assuming gentle threshing), we estimated the recombination between a putative gene for naked grain and a putative gene for fatuoidy, but found no indication of close linkage; the estimated recombination fractions were 0.34 ± 0.09, 0.48 ± 0.08, and 0.45 ± 0.08.
All three of the F2 populations from crosses involving Terra deviated significantly from the 1:2:1 ratio for the number of geniculate awns per spikelet. Of these, only the NO753-2/Terra population deviated significantly from the 3:1 ratio (Table 1). The NO753-2A/Tibor F2 population did not deviate significantly from the 1:2:1 ratio for awn number (Table 1).
Overall, the F2 results are not consistent with the segregation of a single codominant gene for either character. Single-locus models in which the expression of the dominant gene is variable (i.e., some plants with all naked grain and some mosaic plants, or some awnless plants and some plants with one awn per spikelet) seem more appropriate, but there were some cases of significant deviation from the 3:1 ratios expected for these models. For examination of hull adherence, inclusion of intermediate grains with hulled grains (i.e., gentle threshing) seemed to give more consistent ratios than inclusion of intermediate grains with naked grains.
F2:3 Progeny Tests for the Fatuoid Character
On the basis of the results obtained in the F2 generation, one possible model for the inheritance and expression of fatuoidy involves a single dominant gene, with variable expression. Under this model, one would expect that most awnless plants are homozygous nonfatuoids (but a few might be heterozygotes), that plants with one awn per spikelet are either homozygous or heterozygous nonfatuoids (with more heterozygotes than homozygotes), and that most plants with two or more awns per spikelet are homozygous fatuoids.
Under these expectations, most F2:3 families derived from awnless F2 plants would be fixed for nonfatuoidy. Most of the plants in these families would be awnless but some would have one awn per spikelet. However, a small proportion of the F2:3 families derived from awnless plants might segregate for fatuoidy, with about one in four of their plants bearing two awns per spikelet. Our data met these expectations. In 34 of 53 F2:3 families derived from awnless F2 plants, over 90% of the F3 plants were awnless and none had two or more awns per spikelet. In two other families derived from awnless plants, over 80% of the plants were awnless. Both of these families included some plants with two or more awns per spikelet (a ratio of 53:2:9 in one NO753-2/Marion family and a ratio of 46:5:2 in one NO753-2/Terra family). Among the other 17 F2:3 families derived from awnless plants, 16 had sufficient F3 plants with two or more awns per spikelet to provide a satisfactory fit to a 3:1 ratio. The remaining family had an excess of plants with two or more awns per spikelet and few plants with one awn per spikelet (a ratio of 30:6:22 in one NO753-2A/Marion family).
Under the same model, most F2:3 families derived from F2 plants with one awn per spikelet should segregate with a 3:1 ratio, but some would be fixed for nonfatuoidy. The first of these expectations was met, with 40 of the 55 F2:3 families derived by self-pollination of F2 plants with one awn per spikelet exhibiting no significant deviation from the expected 3:1 ratio. The second expectation was not met, with only two families consisting predominantly of awnless plants (a 63:6:9 ratio and a 37:10:4 ratio). Three of the remaining families consisted predominantly of plants with two or more awns per spikelet (i.e., over 90%). For the other 10 families, the observed ratios had no obvious similarity to any expected ratio, and varied considerably from family to family. These results suggest that while most plants with one awn per spikelet may behave as though they are segregating for a single dominant gene, the true genetic model may be considerably more complex.
In the Marion crosses, seven families derived from F2 plants with one awn per spikelet gave a satisfactory fit to the 1:2:1 ratio. All of these were derived from hulled F2 plants. In the same crosses, there were four families that deviated from both the 1:2:1 ratio and the 3:1 ratio. All of these were derived from naked-grained F2 plants. These observations provide some support for the idea that simple genetic models for fatuoidy may be more appropriate in hulled material than in naked-grained material.
Finally, if plants with two or more awns per spikelet are homozygous for a recessive allele conditioning fatuoidy, self-pollination of these plants would give rise to fatuoid progeny only. The expected phenotypic ratio within F2:3 families derived from F2 plants with two or more awns per spikelet would be 0:0:1. No statistical tests are possible for this ratio, but the data appear to fit the expectations well; among 3727 F3 plants derived from F2 plants with two or more awns per spikelet, only 126 (3.4%) were awnless and only 150 (4.0%) had one awn per spikelet.
Association between Fatuoidy and Hull Adherence in Naked Oat
To investigate whether the high proportions of naked grain that were observed in NO753-2 and NO753-2A are due to the fatuoid character itself, we tested whether phenotypic classes for awn number differed with respect to their proportions of naked and hulled grain in NO753-2/Terra, NO753-2A/Terra, and NO753-2A/Tibor. In all of these three crosses, F2 plants with two or more awns per spikelet (putative homozygous fatuoids) produced higher proportions of naked grain than plants with fewer than two awns per spikelet (Table 2). Similarly, in F2:3 families derived from F2 plants with one awn per spikelet (i.e., families that may have been segregating at a locus conditioning fatuoidy), plants with two or more awns per spikelet produced higher proportions of naked grain than plants with fewer than two awns per spikelet (Table 2).
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According to these results, selection for the fatuoid character (via selection of plants with two or more awns per spikelet) may be useful to reduce hull adherence problems in naked oat, but selection for three awns per spikelet rather than two cannot be expected to provide any further advantage with respect to hull adherence.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The inheritance of the fatuoid character was of greater interest to us, since it had not previously been investigated in naked oat. In the materials used here, fatuoids were easily recognizable according to the presence of geniculate lemma awns, and we based our investigation of the fatuoid character on phenotypic ratios for the maximum numbers of geniculate awns per spikelet. As with the naked-grained character, observed F2 ratios did not consistently fit those expected for a single gene. Nevertheless, the results of F3 progeny tests demonstrated that phenotypic selection was quite reliable both for fatuoids (via selection of plants with up to two or more geniculate lemma awns per spikelet) and for nonfatuoids (via selection of plants without geniculate lemma awns). Selection of plants with one geniculate awn per spikelet gave less predictable results. Fatuoid naked oat plants were confirmed to have an advantage over nonfatuoid naked oat plants, in that they produced higher proportions of naked grains (i.e., a lower incidence of hull adherence) but we saw no further advantage in selecting for plants with multiple awns per spikelet (vs. only two), suggesting that any relationship between fatuoidy and hull adherence is not directly related to awn number itself.
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ACKNOWLEDGMENTS |
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Received for publication August 31, 2001.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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