Hybrid breakdown and inbreeding depression for seed yield and early growth in 2nd-generation interspecific hybrids of larch (Larix x eurolepis x L.x eurolepis)

Advanced hybridization between first-generation interspecific hybrids is an attractive alternative to breed and mass-produce hybrid seed lots. Nevertheless it faces two major drawbacks, namely hybrid depression and inbreeding depression. Their impact may be highly detrimental on some traits (seed yield) especially for the highest level of consanguinity, but much less on some others (seed quality and early growth) as shown in this study for hybrid larch (Larix x eurolepis).

Context Mass-production of 1st-generation larch hybrids (Larix x eurolepis) in interspecific hybridization orchards is problematic. Alternatives are searched such as advanced-generation orchards combining 1st-generation hybrid clones.
Aims Our aim was to investigate how important hybrid and inbreeding depressions can be in 2nd-generation hybrids with a special focus on reproductive success (and early growth) and their consequences on the success of advanced-generation hybridization seed orchards.
Methods Second-generation hybrids were created by control crosses among 18 1st-generation (F1) hybrid clones with 3 different levels of relatedness (OC (outcrossed), HS (half-sib) and FS (full-sib)). Seed yield and quality and growth in nursery were assessed for the 3 groups and compared with related F1 crosses. Hybrid breakdown and inbreeding depression were then estimated.
Results F1 hybrids are fertile and mating among them is feasible. Nevertheless hybrid depression is observed when passing to 2nd-generation hybrids for crossing success rate (− 20%) and for seed yield (> 50% less filled seed/cone). Hybrid depression is reinforced by inbreeding and more severely with increasing consanguinity levels: negligible for HS, a supplementary drop of over 50% of seed yield is observed for FS. In contrast, seed quality traits as well as first-year growth are little impacted by both hybrid and inbreeding depressions. Nevertheless a great variation in inbreeding depression level exists among mother clones. Overall, total depression overpasses 70% for the FS parent group but is limited to less than 34% for OC parents for a synthetic trait (number of germinated seed/cone) and remains negligible for growth.
Conclusion Advanced hybridization is possible with larch but with a more or less severe impact on reproductive success depending on level of relatedness among the parents. Growth seems much less impacted than reproduction and this is a promising result. The proper choice of 1st-generation parents is unavoidable for success of 2nd-generation seed orchards.

Interspecific hybridization, Hybrid breakdown, Inbreeding, Reproduction, Seed, Height

Pâques, L.E. Annals of Forest Science (2019) 76: 69. https://doi.org/10.1007/s13595-019-0853-8

For the read-only version of the full text: https://rdcu.be/bJN6x

Data availability
The datasets generated and/or analysed during the current study are available in the Portail Data INRA repository (Pâques 2019), at https://doi.org/10.15454/AMNDTO.

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