Abstract
Polyploidisation is a key source of diversification and speciation in plants. Most researchers consider sexual polyploidisation leading to unreduced gamete as its main origin. Unreduced gametes are useful in several crop breeding schemes. Their formation mechanism, i.e., First-Division Restitution (FDR) or Second-Division Restitution (SDR), greatly impacts the gametic and population structures and, therefore, the breeding efficiency. Previous methods to identify the underlying mechanism required the analysis of a large set of markers over large progeny. This work develops a new maximum-likelihood method to identify the unreduced gamete formation mechanism both at the population and individual levels using independent centromeric markers. Knowledge of marker-centromere distances greatly improves the statistical power of the comparison between the SDR and FDR hypotheses. Simulating data demonstrated the importance of selecting markers very close to the centromere to obtain significant conclusions at individual level. This new method was used to identify the meiotic restitution mechanism in nineteen mandarin genotypes used as female parents in triploid citrus breeding. SDR was identified for 85.3% of 543 triploid hybrids and FDR for 0.6%. No significant conclusions were obtained for 14.1% of the hybrids. At population level SDR was the predominant mechanisms for the 19 parental mandarins.
Highlights
Polyploidisation is a key source of diversification and speciation in plants
The method proposed by Cuenca et al.[37] was successfully applied in populations of 2n ovules of ‘Fortune’ mandarin and ‘Nules’ clementine, and it was concluded that Second-Division Restitution (SDR) was the main restitution mechanism and that partial chromosome interference occurs[36,37]
LOD scores greater than 3 or greater than 2 were considered as thresholds indicating that SDR was the mechanism involved in the single unreduced gamete formation, whereas LODs below 23 indicate that First-Division Restitution (FDR) was the underlying mechanism; for LOD scores between 23 and 3, we considered that the mechanism could not be determined significantly
Summary
Polyploidisation is a key source of diversification and speciation in plants. Most researchers consider sexual polyploidisation leading to unreduced gamete as its main origin. Unreduced gametes are useful in several crop breeding schemes Their formation mechanism, i.e., First-Division Restitution (FDR) or Second-Division Restitution (SDR), greatly impacts the gametic and population structures and, the breeding efficiency. This work develops a new maximum-likelihood method to identify the unreduced gamete formation mechanism both at the population and individual levels using independent centromeric markers. Simulating data demonstrated the importance of selecting markers very close to the centromere to obtain significant conclusions at individual level. This new method was used to identify the meiotic restitution mechanism in nineteen mandarin genotypes used as female parents in triploid citrus breeding. Failure of the first (FDR) or second (SDR) divisions leads to the formation of restitution nuclei with an unreduced chromosome number.
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