IUFRO Breeding theory and progeny testing

Maritime pine (Pinus pinaster) is the first plantation tree species in France where it represents 10% of the forest area and 24% of wood harvest. The South West of France (Aquitaine) is the main production region with 1 million ha of cultivated maritime pine forest, planted or sowed since 1850. The productivity, from an average of 10 m3/ha/y, can reach 20 to 25 m³/ha/y in current increment on best sites; the rotation age is around 45y and is decreasing with improved varieties. Today 8.5 millions m³ per year are harvested and largely processed locally into 60% sawtimber, and 40% industrial wood.

The breeding programme started in the 60s, when the early provenance trials had already shown the superiority of the local Landes provenance for growth and cold resistance (Aquitaine being the most northern region of the species natural distribution, localized around the Mediterranean basin). The local resources were then chosen to build up a breeding population, despite their form defects: trunk flexuosity and poor branch quality. A few hundreds of plus trees were phenotypically selected in the maritime pine forests on the coastal sand dunes of Aquitaine, based on growth measurements and stem form visual scoring of the candidate trees and their 30 closest neighbours (even aged forests). This first step of selection proved to be efficient for growth and stem straightness improvement, based on progeny test when plus trees were compared with non-selected material after 10 years old. Additionally, about 500 clones from Corsica provenance were selected in provenance trials located in Aquitaine, based on growth, stem straightness, branch quality, pyralis resistance (Dioryctria sylvestrella), and cold resistance. The objective of this second population is to produce improved Landes x Corsica varieties for better stem straightness and branch quality.

Development of the breeding programme followed a classical design of recurrent selection with a main population composed of the Landes plus trees. In the first two cycles of selection, factorial mating designs with 4 to 6 crosses per parent, or hierarchical (nested) mating designs with 2 crosses per parent were used to produce the next generations, while parents were also evaluated through common polycross testing. Selection was based on an individual-family index, with total height, DBH and stem deviation to verticality at 10 years old as selection criteria. With this strategy, 3 generations of the main population, more than 4500 selected individuals, 5000 progenies tested over 500 ha of trials were obtained, and 3 generations of seed orchards were produced.

For economical and technical reasons, the deployment strategy for maritime pine varieties in Aquitaine is mainly based on open pollinated seed orchards. The first generation seed orchards proved to realize a genetic gain of 10 to 15 % on volume and stem straightness at 15 years old (average age), as showed by results obtained over a network of seed orchards and non-improved material comparison trials. For the second generation seed orchards, actually commercialized, the expected genetic gain was estimated from progeny trials at 13 years old to 30 % on both criteria over non improved material. In 2003, 70% of reforested areas in Aquitaine were planted, from which 100% are made with this second generation improved seed source. The third generation seed orchards are now being planted over 180 ha, and they should enter in production by 2010-2015.

For the next generations of the breeding population, the focus is on a better management of gene diversity while being able to improve genetic gain trough selection of about 30 non related clones at each generation (necessary to produce OP seed orchards). Some 10 sublines are being constituted within the breeding population, based on pedigrees and breeding values. Status number is used as an indicator of genetic diversity. Incomplete factorial mating designs will be used, while polycross testing is performed for parents ranking. Trials are replicated on 3 contrasting sites, with usually single tree plots and 35 reps per site. Including new selection criteria is another focus: studies on wood quality, drought tolerance and pests and diseases resistance are going on.

Biotechnologies for maritime pine are under study as well: latest advances on molecular markers include candidate gene screening (SNPs) in natural populations for wood formation and drought resistance, and validation by association studies. Molecular markers (nuclear microsatellites) are also used in an experiment for detecting pollen contamination in OP seed orchards. Embryogenesis from immature seeds is worked on for some good progenies. Promising results were obtained in these different approaches and thus some new technologies could be incorporated in the next years to improve our strategy in the breeding programme.

Author: Annie Raffin INRA (Institut National de la Recherche Agronomique), UMR Biogeco (Biodiversity, Genes, Ecosystem) 69, route d’Arcachon – 33612 Cestas Cedex – France.