The Metastable Nature of Paramutable R Alleles in Maize. IV. Parallel Enhancement of R Action in Heterozygotes with r and in Hemizygotes.
The Metastable Nature of Paramutable R Alleles in Maize. IV. Parallel Enhancement of R Action in Heterozygotes with r and in Hemizygotes.
复制标题
玉米副突变 R 等位基因的亚稳态性质。
作者：
E. Styles;R. Brink
在特定条件下，ARAMutation导致基因功能状态的可预测和可遗传的变化（见综述，BRINK，STYLES和AXTELL 1968）。在玉米调节花青蛋白形成中，R基因座的某些等位基因在通过具有RSt（斑点糊粉）的杂合子后糊粉着色特性不变地降低。在具有隐性r（无色糊粉）等位基因的杂合体中，相同的等位基因的糊粉着色作用增强。因此，通过R-RSt或R-r杂合子传递它们，可遗传地抑制或遗传地增强paramutable R基因的作用。这些因素与场所本身密切相关。同样，使RSt和其他明显的R等位基因降低R作用的因素与R基因座密切相关。本文关注的是与r等位基因相关的因素，可能是负责增强的paramutable R等位基因在R r杂合子的测试结果。结果表明，在半合子中也可以发生可调R等位基因的增强，即，当与R基因座缺陷相反时，如在R r杂合子中。因此，似乎r等位基因是无定形的，相对于副突变的变化，和增强的副突变的R等位基因可以自主发生的意义上说，它发生在基因组中的等位基因区域的情况下。W22。在每次测交时，根据标准等级对每个测交穗的每个表现R ′抑制形式的W23 rgrg进行评分，标准等级定义了从无色（1类）到完全着色（7类）的7个等级的糊粉斑驳。如此获得的平均值被用作植物中R等位基因的着色潜力的量度。实验设计允许将R W植物（红色花药）与Rg/r-x（绿色）区分开，并且所有杂交都是在具有相似成熟度的一个和一个花药植物中进行的，可能的变化来自成熟效应。Rgrr Rg/r-xI植株的测交结果与Rgrr Rg/r-xI植株相似。称为R的“副突变形式”或“R”。R'基因的作用水平通常比标准形式低得多，因为与RSt的杂合性导致R作用的显著抑制。然而，从一个单一的R Rst植物产生的配子中的R基因，相对于它们所经历的抑制量而变化。可能的极端情况包括，在极低频率下，R基因以某种方式“逃脱”副突变，因此在表达上与标准形式相似（BROWN 1966），以及在R基因座内发生重组以形成强副突变等位基因的情况，该等位基因调节近乎无色的测交表型（ASHMAN 1965）。在启动R'谱系中使用修改的程序以检测在第一代中这些罕见事件之一的可能选择。来自rr/r-xl x RgRst交配产生的每个穗的籽粒基于糊粉层中表现出的斑点程度被分成两个相等大小的群体。每个种群的随机样本被播种，以建立两个独立的谱系，“黑暗”和“光明”，从每个耳朵。此后，在任何时候都不进行基于表型的选择。一个逃避副突变的R RSt植物中的R基因预计只会在一个“黑暗”的血统，一个近无色的重组应该只发生在一个“轻”的血统。
ARAMUTATION results in a predictable and heritable change in the func-tional state of a gene under specified conditions (see review, BRINK, STYLES and AXTELL 1968). Certain alleles of the R locus in maize conditioning antho-cyanin formation invariably are reduced in aleurone pigmenting properties following passage through a heterozygote with RSt (stippled aleurone). The same alleles are enhanced in aleurone pigmenting action in heterozygotes with a recessive r (colorless aleurone) allele. The action of paramutable R genes, therefore, can be heritably repressed or heritably enhanced by passing them through R RSt or R r heterozygotes, respectively. The factors involved intimately locus itself. Likewise the factors that enable RSt and other overtly R alleles to a reduction in the action of a R are closely associated with the R locus. This article is concerned with the results of tests for factors associated with r alleles that may be responsible for the enhancement of paramutable R alleles in R r heterozygotes. The results show that enhancement of a paramutable R allele can occur as well in hemizygotes, i.e., when in opposition with a deficiency for the R locus, as in R r heterozygotes. Thus it appears that r alleles are amorphic with respect to paramutational change, and that enhancement of a paramutable R allele can occur autonomously in the sense that it takes place in the absence of an allelic region in the genome. W22. at each testcrossing each W23 rgrg of showing the R’ repressed form of R) phenotype was scored from each testcross ear against a standard scale defining seven grades of aleurone mottling from colorless (class 1) to fully pig- mented (class7). The mean value so obtained was taken as a measure of the pigmenting potential of the R allele in the plant The experimental design allowed R W plants (red anthers) to be distinguished from Rg/r-x, (green in the and all crosses made in painvise one and one anthered plant of similar maturity, possible variations arising from maturation effects. testcrossed and in a similar to for the Rgrr Rg/r-xI plants. referred to as the “paramutant form” of R, or “R”’. R’ genes usually have a much lower level of action than the standard form because of the marked repression of R action that follows heterozygosity with RSt. R genes in gametes resulting from a single R Rst plant, however, vary with respect to the amount of repression they have undergone. Possible extremes include, at very low frequencies, R genes that have somehow “escaped” paramutation and thus are similar in expression to the standard form (BROWN 1966), and cases in which a recombination has OCcurred within the R locus to form a strongly paramutagenic allele that conditions a near- colorless testcross phenotype (ASHMAN 1965). A modified procedure was used in initiating R’ lineages to detect a possible selection of one of these rare events in the first generation. Kernels from each of the ears resulting from the rr/r-xl x RgRst matings were divided into two equal sized populations on the basis of degree of mottling exhibited in the aleurone. Random samples of each population were sown to establish two separate lineages, “dark” and “light,” from each ear. Thereafter no selection on the basis of phenotype was made at any time. An R gene escaping paramutation in the R RSt plant would be expected only in a “dark” lineage; a near-colorless recombinant should occur only in a “light” lineage.