Evaluation of polyembryony for genetic resources and efficacy of simple sequence repeat markers for the identification of nucellar and zygotic embryo-derived individuals in citrus

Abstract

Many citrus cultivars have the polyembryony trait that develops many nucellar embryos alongside a single zygotic embryo in an individual seed by sporophytic apomixis. This unique botanical trait hinders citrus breeding by genetic hybridization and affects breeding efficiency and cost. Techniques to efficiently identify nucellar and zygotic individuals in citrus are still very limited. For a systematic and targeted citrus breeding program, we collected 101 citrus genetic resources and determined their embryo types, which revealed 22 monoembryo, 54 polyembryo, and 25 mixed types. We also developed 17 simple sequence repeat (SSR) markers showing polymorphism among the genetic resources from the public resources and our own comparative genome analysis. Seventeen SSR markers detected a total of 181 alleles, ranging from 5 to 16 alleles per locus. The average polymorphism information content value was 0.67, ranging from 0.43 to 0.84. Genetic cluster analysis based on similarity matrices of alleles revealed that several genetic resources of the genus Citrus were fragmented and/or scattered throughout the entire dendrogram, not forming unique groups, due to frequent natural or intended genetic crossings. Application of these polymorphic SSR markers to F 1 individuals derived from several genetic crosses using polyembryonic citrus cultivars as a female parent revealed that the polyembryony trait decreased the breeding efficiency due to the poor occurrence rate of zygotic individuals. Therefore, our results suggest that identification of nucellar and zygotic embryo-derived F 1 individuals using SSR markers as a genotyping technology may be a powerful tool for establishing a systematic molecular breeding program in citrus.

Cytogenetical Changes among Polyembryonic (PEm) and Non-PEm Maize Plants

“…However, the tetrads found in PEm and Non-PEm maize were normal. These results may be interesting in other agricultural commodities, such as citrus cultivars which develop many nucellar embryos alongside a single zygotic embryo from a single seed [22].…”

Development of Indel markers for the selection of Satsuma mandarin (Citrus unshiu Marc.) hybrids that can be used for low-cost genotyping with agarose gels

“…However, as these markers are not intended to detect specific loci showing polymorphism, to improve reproducibility, they must be converted to a single marker [e.g., a sequence tagged site (STS)] that detects one locus by designing specific primers. In recent years, simple sequence repeat (SSR) markers have been used successfully to identify zygotic embryos (Ruiz et al 2002;Carlos de Oliveira et al 2002;Rao et al 2008;Shareefa et al 2009;Jannati et al 2009;Yildiz et al 2013;Woo et al 2019). However, polymorphisms detected in most of the SSR markers were short in DNA size, and this requires either advanced and expensive equipment, such as a capillary DNA sequencer, or not complicated but time-consuming procedures, such as polyacrylamide gel electrophoresis.…”