Growth evaluation of the second-generation core population open-pollinated progeny of loblolly pine and relatedness selection based on chloroplast SNPs

To evaluate the growth of the second-generation core population open-pollinated progeny of loblolly pine, screen for superior families and individual trees, analyze the paternal relatedness of superior individual trees using chloroplast SNPs, and conduct a secondary selection of these superior trees to identify candidates for the third-generation core population.

Using the second-generation core population open-pollinated progeny of loblolly pine in Guangdong Province as research objects, a survey of growth traits at the age of 6 years was conducted. Genetic evaluation was performed using SAS and R software. A combined selection approach was used to identify 35 superior individual trees for SNP marker development and relatedness analysis.

1) Significant differences in growth traits were observed among families, with performance exceeding that of the controls. The average height, diameter at breast height (DBH), and volume of the progeny forest were 6.56 m, 14.67 cm, and 0.055 7 m³, respectively, which were higher than those of CK1 (6.26 m, 13.00 cm, 0.042 7 m³) and CK2 (6.31 m, 13.17 cm, 0.042 6 m³); 2) Based on volume breeding value ranking, 25 superior families were selected with average height, DBH, and volume of 6.65 m, 15.34 cm, and 0.061 0 m³, respectively. Using a combined selection method, the top two individuals with the best overall performance were selected from the top 10 families, and the best individual was selected from families ranked 11th to 26th. Family Ⅲ-68-4 was excluded due to the lack of superior individuals. A total of 35 superior individual trees were selected; 3) Chloroplast genome sequencing of the superior individual trees revealed their paternal relatedness. To control relatedness, the number of individuals potentially originating from the same paternal parent was limited to two. After adjusting the number of superior individuals while maintaining the number of superior families, 31 superior individual trees were finally selected.

The 35 superior individual trees had an average height of 7.27 m, DBH of 19.63 cm, and volume of 0.101 5 m³, with an average volume genetic gain of 17.51%. Considering their relatedness, the 31 selected superior individual trees are recommended for inclusion in the third-generation core population of loblolly pine.