Genomic Breeding for Diameter Growth and Tolerance to Leptocybe Gall Wasp and Botryosphaeria/ Teratosphaeria Fungal Disease Complex in Eucalyptus grandis

Abstract

Eucalyptus grandis is one of the most important species for hardwood plantation forestry around the world. At present, its commercial deployment is in decline because of pests and pathogens such as Leptocybe invasa gall wasp (Lepto), and often co-occurring fungal stem diseases such as Botryosphaeria dothidea and Teratosphaeria zuluensis (BotryoTera). This study analyzed Lepto, BotryoTera, and stem diameter growth in an E. grandis multi-environmental, genetic trial. The study was established in three subtropical environments. Diameter growth and BotryoTera incidence scores were assessed on 3,334 trees, and Lepto incidence was assessed on 4,463 trees from 95 half-sib families. Using the Eucalyptus EUChip60K SNP chip, a subset of 964 trees from 93 half-sib families were genotyped with 14,347 informative SNP markers. We employed single-step genomic BLUP (ssGBLUP) to estimate genetic parameters in the genetic trial. Diameter and Lepto tolerance showed a positive genetic correlation (0.78), while BotryoTera tolerance had a negative genetic correlation with diameter growth (-0.38). The expected genetic gains for diameter growth and Lepto and BotryoTera tolerance were 12.4, 10, and -3.4%, respectively. We propose a genomic selection breeding strategy for E. grandis that addresses some of the present population structure problems.

Keywords: Botryosphaeria dothidea; Eucalyptus grandis; Leptocybe invasa; Teratosphaeria zuluensis; genetic correlation; ssGBLUP.

FIGURE 1

Historical overview of E. grandis breeding in South Africa, including a transition from government to private breeding and introduction of major pest and pathogens. The trial series timeline, as well as the generational timeline, are shown. Selection strategies are noted for each trial series, shifting from timber to pulp and paper related traits, as well as pest and disease tolerance. Selection refers to the selection of phenotyped individuals based on their breeding values, whereas evaluation refers to the selection of individuals based on visual screening without breeding values.

FIGURE 2

Geographical representation of the trial sites in the KwaZulu Natal province, South Africa. The region has a sub-tropical climate. The distance (straight line) between Mtunzini and Nyalazi is 112 km. The details of the environmental conditions are in Supplementary Table 1. Darker shades of green indicate nature reserves.

FIGURE 3

Symptoms and incidence scores of Leptocybe invasa (Lepto). (A) Score 4 – No evidence of an attack on the leaf midrib or petiole, (B) Score 3 – Evidence of attack on the leaf midrib or petiole without galls (indicated by red arrows), (C) Score 2 – Evidence of attack on the leaf midrib or petiole with galls, and (D) Score 1 – Evidence of a lethal outcome of an attack on the leaf midrib or petiole with galls.

FIGURE 4

Symptoms and incidence scores for Botryosphaeria/Teratosphaeria stem disease complex (BotryoTera). (A) A score of 6 represents trees with no spots/cracks or redness. (B) A score of 5 represents trees with T. zuluensis spots with redness. (C) A score of 4 is given for trees with B. dothidea cracks with redness. (D) A score of 3 shows a tree with T. zuluensis spots and B. dothidea cracks with redness. (E) A score of 2 represents trees with heavy T. zuluensis spots and B. dothidea cracks with redness. (F) A score of 1 represents trees with heavy T. zuluensis spots and B. dothidea cracks with redness and cankers.

FIGURE 5

Marginal trait means with error bars indicating the 95% confidence interval. (A) Mean diameter growth (cm) for families in the three sites. (B) The mean Lepto tolerance score for families in the three sites. (C) The mean BotryoTera tolerance score for families in the three sites. (D) Mean diameter growth (cm) for families in the three generations. (E) The mean Lepto tolerance score for families in the three generations. (F) The mean BotryoTera tolerance score for families in the three generations. Student t-test was performed to assess the significant difference between the means, p < 0.05 (*) and p < 0.001 (***).

FIGURE 6

Proposed breeding strategies to improve diameter growth under pest and pathogen pressures. (A) Traditional field-based multivariate selection strategy whereby diameter growth (genetically correlated with Lepto tolerance) is the target trait. BotryoTera tolerance selections are made within top-ranked diameter growth families to produce open-pollinated (OP) families for the next generation. (B) Proposed non-field-based serial selection strategy in which Lepto tolerance and BotryoTera tolerance are scored after successive (6 and 12 months) controlled infestation and inoculations, respectively. Candidate seedlings from within these tolerant families are cloned and used for flower induction (Set C) and generation of CP families for the next generation. Another set of candidate clones is used to validate the Lepto and BotryoTera tolerance (Set A). The third set is then planted in field clonal trials for diameter growth (Set B). Accurate phenotypes from the clonal material and genome-wide genotyping of the clones create an opportunity to train a genomic selection model that can reduce (pink arrows) the need for expensive pest and disease phenotyping in the next generation.