A small-leaf, spear-shaped leaflet peanut (Arachis hypogaea L.) mutant was first reported by Bhide and Desale (1970) and later by Desale (1987). This spear-shaped mutant was selected from a field increase of the semi-spreading cultivar 'Kopargaon No. 1' at Jalgaon, India during 1964. The mutant was subsequently crossed with Kopargaon No. 1, and the F1 plants had normal leaves which suggest that this mutant was recessive. F2 segregation fit a 3:1 ratio for normal and spear-shaped leaflet plants, respectively. The study by Bhide and Desale (1970), showed that the small-leaf, spear-shaped mutant was controlled by recessive monogenic inheritance.
During 2012, a new Spear-shaped Leaf mutant was isolated from the runner-type peanut cultivar 'Georgia-06G' (Branch, 2007). This mutant likewise had small spear-shaped leaflets with each leaflet tapering to a point at the top and bottom and the widest section being in the middle which gives the appearance of a spearhead shape (Figure 1). The objective of this study was to determine the inheritance of the present Spear-shaped Leaf mutant.
Materials and Methods
Three cross combinations were made in the greenhouse between Georgia-06G x Spear-shaped Leaf, 'OLin' (Simpson et al., 2003) x Spear-shaped Leaf, and 'Georgia-04S' (Branch, 2005) x Spear-shaped Leaf during 2013. F1, F2, and F3 populations were grown during 2014, 2015, and 2016, respectively.
Each year, seed were space-planted 30.5 cm apart in two rows with variable length depending upon number of seed x 1.8 m wide beds on a Tifton loamy sand soil type (fine-loamy, siliceous, thermic, Plinthic Kandidult) at the agronomy research farm near the University of Georgia, Coastal Plain Experiment Station, Tifton Campus. Recommended cultural practices with irrigation were followed throughout the growing seasons.
Individual plants were harvested near optimum maturity based upon number of days after planting and above-ground plant appearance. After harvest, peanut pods were dried with forced warm air to approximately 6% moisture content before weighing and shelling.
Phenotypic classification was based on individual plants before digging and inverting. Segregation data was analyzed by chi-square analysis for goodness-of-fit (P≤0.05) to expected genetic ratios (Strickberger, 1968). Least significant difference (LSD) t-test was used to compare the ten plant average between F2:3 spear-shaped and normal leaf homozygous genotypes for mainstem height, leaflet length and width, pod weight, sound mature kernels (SMK), and 100 SMK weight.
Results and Discussions
Each F1 plant from all three crosses had an intermediate Spear-shaped Leaf. This indicates that the Spear-shaped Leaf trait is incompletely dominant to normal leaves which is different from the first report by Bhide and Desale (1970).
The F2 segregation from the two cross combinations (Georgia-06G x Spear-shaped Leaf and Georgia-04S x Spear-shaped Leaf) showed an acceptable fit for a 1 Spear-shape to 2 Intermediate to 1 Normal leaf expected ratio, respectively (Table 1). However, the F2 segregation from the OLin x Spear-shaped Leaf cross combination did not fit the 1:2:1 ratio. This deviation from the expected ratio could be due to several albino seedlings observed in this infraspecific cross between subspecies (ssp. fastigiata and ssp. hypogaea) which died shortly after emergence. Because of this infraspecific cross combination, the total chi-square value was also found to be significantly different at P≤0.05, however the summed and homogeneity chi-square values were each found acceptable for the 1:2:1 expected genetic ratio. These F2 results also disagree with the earlier report for the small-leaf, spear-shaped leaflets (Bhide and Desale, 1970).
F3 results confirmed a single incomplete dominant inheritance model with a 1:2:1 expected ratio from F2:3 heterozygous intermediate Spear-shaped Leaf plants (X2 = 2.400, P = 0.25 - 0.50). Likewise, the Spear-shaped Leaf plants and normal shaped leaf plants bred true-to-type in the F2:3 population from Georgia-06G x Spear-shaped Leaf cross combination as was expected. These F2:3 homozygous spear-shaped individual plants had taller mainstem, narrow leaflet width, reduced pod weight, and lower sound mature kernel (SMK) percentages compared to the F2:3 homozygous normal leaf plants resulting from this same closely related cross combination at maturity (Table 2). Early in the growing season, it also was observed that the spear-shaped F2:3 plants appear to have more thrips damage than the normal plants.
These findings strongly suggest that the inheritance of the present Spear-shaped Leaf mutant is controlled by a single incompletely dominant gene, designated (SpL). This data also disagrees with the past recessive monogenic inheritance for the spear-shaped leaflet trait reported by Bhide and Desale (1970) and Desale (1987). It would have been interesting to make allelism tests between these two spear-shaped mutants, unfortunately there were nearly 50 years between the occurrences of these two leaf mutants.
M. V Bhide, and S. C Desale (1970). A small leaf mutant in groundnut. Poona Agric. Coll. Mag 59: 113- 114.
W. D Branch, (2005). Registration of 'Georgia-04S' peanut. Crop Sci 45: 1653- 1654.
W. D Branch, (2007). Registration of 'Georgia-06G' peanut. J. Plant Reg 1: 120.
S. C Desale, (1987). Genetic studies in groundnut. J. Maharashtra Agric. Univ 12: 126- 127.
C. E., Simpson, M. R Baring, A. M Schubert, H. A MeLouk, Y Lopez, and J. S Kirby (2003). Registration of 'OLin' peanut. Crop Sci 43: 1880- 1881.
Strickberger, M. W 1968 Chap 8. Probability and statistical testing Pp 126- 152 In: Genetics The Macmillan Co., New York, NY.
- Professor, University of Georgia, Department of Crop and Soil Sciences, Coastal Plain Exp. Sta., 2360 Rainwater Road, Tifton, GA 31793-5766. [^] *Corresponding author email: firstname.lastname@example.org