Parent-Progeny Regression in Indiangrass: Inflation of Heritability Estimates by Environmental Covariances

Families of two indiangrass [Sorghastrum nutans (L.) Nash] populations were used in this study with a family plot consisting of a parental clone and four half-sib progeny_ The experimental design was a randomized complete block with two replications. The regression of offspring on the parent in the same plot was used to obtain heritability estimates in which environmental covariances might be expected to inflate the parent-offspring covariance. The regression of offspring in one replication with its parent in another replication, and vice versa, was used to obtain heritability estimates whereby environmental covariances should be minimized. Heritability estimates for heading date and plant height, which had high heritabilities, were similar for the two estimation techniques. Heritability estimates for forage yield and in vitro dry matter digestibility (IVDMD) appeared to be influenced by environmental covariances. Mean heritability estimates for forage yield and IVDMD for the three sets of data analyzed were 0.64 and 0.94 respectively, when parents and offspring in the same plot were used in the regression analyses and 0.28 and 0.42 respectively, when parents and offspring in different plots were used. Heritability estimates for forage protein content averaged 0.49 and were similar for both methods of estimation. Additional index words: Sorghastrum nutans, Forage yield, Forage quality, In vitro digestibility, Genetic covariances. 1 Contribution of the Dep. of Agronomy, Univ. of Nebraska, and AR, SEA, USDA. Published as Paper No. 5821, Journal Series, Nebraska Agric. Exp. Stn. Received 26 Sept. 1979. • Research agronomist, AR, SEA, USDA; George Holmes professor of Agronomy, Univ. of Nebraska; and supervisory research geneticist, AR, SEA-USDA, Univ. of Nebraska, Lincoln, NE 68583. R EGRESSION of offspring on parents or on one parent is a method of estimating heritability commonly used by plant breeders. The covariance between a parent and its offspring produced by random mating is an estimate of one-half the additive genetic variance in a population (3, 5). The mean of several offspring is usually used as the offspring value. The covariance between offspring and parents is divided by the phenotypic variance of the parents to obtain the regression coefficient. Because the covariance between offspring and parent is only one-half the additive genetic variance, the regression coefficient is multiplied by two to obtain an estimate of narrow sense heritability (5). Regression of offspring on parents to estimate heritability is based on the following assumptions (3, 4): a) normal diploid and solely Mendelian inheritance, b) no environmental correlations among relatives, c) population in linkage equilibrium or no linkage among genes controlling the traits studied, d) the relatives are noninbred, e) the genetic population is mating at random. Heritability estimates obtained by parent-progeny regression are often used because the estimates obtained are valid both when the parents are selected on some basis and when they are chosen at random from a population (5). The purpose of this paper is to demonstrate the inflationary effect that environmental covariances might have on heritability estimates if the second assumption, i.e., no environmental correlations among relatives, is violated. In 1973, we began a quantitative genetic study on indiangrass, Sorghastrum nutans (L.) VOGEL ET AL.: PARENT-PROGENY REGRESSION IN INDIANGRASS 581 N ash. Our results from this study were used to estimate the effect of environmental covariances on heritabilities based on parent-progeny regression. Indiangrass is a warm-season grass native to the prairies and plains of the United States (9) where it is used in pastures and rangelands for summer grazing. Limited genetic research on this grass has been reported although several varieties have been released. In our study we used two varieties that are adapted to the central Great Plains. 'Holt' was developed by mass selection from ecotypes collected in northeast Nebraska while 'Oto' was developed by mass and progeny selection from ecotypes collected in southeast Nebraska and northeast Kansas (7). MATERIALS AND METHODS In 1973. space-transplanted populations of a Holt X Oto synthetic and Oto indiangrass were sampled at panicle emergence for in vitro dry matter digestibility percentage (IVDMD) and for crude protein. Each nursery contained approximately 1.000 plants_ The Holt X Oto synthetic was established from seed harvested from Holt plants exposed to Oto pollen in an open-pollinated Holt X Oto crossing block. Because Holt flowers 20 days earlier than Oto. it is doubtful if any crossing occurred and the Holt X Oto synthetic will be referred to as Holt. Plants in both nurseries were 2 years old when sampled. The Holt population was located at Lincoln. Nebraska. while the Oto population was located at Mead. Nebr. which is approximately 55 km northeast of Lincoln. In 1973. seed from open-pollinated plants was harvested from plants sampled from the two populations. In 1974. a twice-replicated. space-transplanted half-sib progeny nursery was established at the Mead Field Laboratory with seedlings grown from open-pollinated seed from 41 Holt and 150 Oto parents that represented the high and low ends of the range in IVDMD. protein percentage. and heading date for the two populations. In 1975. two ramets (propagules) of the parental clones similar in size to the seedlings established the p'revious year were transplanted into the same nursery. A famIly plot consisted of a five-plant row with a parental ramet and four progeny seedlings. The parental ramet was at the end of the plot. Rows and plants within rows were spaced 1.1 m apart. The experimental design was a randomized complete block. Holt and Oto families were randomized together in the . / same nursery. but because of maturity differences were analyzed .