Application of a MITE Citrus apomixis marker in the Australian rootstock breeding program

Abstract

Australia's citrus breeding efforts are small by international standards, and unashamedly focused on conventional approaches. Molecular markers had not yet been used in the program because they failed to meet our four essential criteria of being: linked to a trait of economic significance; technically difficult to phenotype by conventional methods; temporally difficult to phenotype by conventional methods and; likely controlled by a simple genetic mechanism. This situation changed dramatically with the 2017 publication of a miniature inverted-repeat transposable element (MITE) marker that co-segregated with the Citrus apomixis trait. This met all of the above four criteria and was quickly verified on local germplasm. Application of this MITE marker is now a standard screening procedure in our rootstock breeding research. An extensive network of field rootstock trials is used to identify parents for rootstock breeding, and the resulting segregating populations are nursery-screened within 18 months of sowing for tolerance to phytophthora, resistance to CTV, and salt exclusion using conventional screening techniques. Hybrids that survive this screening are then assessed for apomixis using the MITE marker. Monoembryonic and polyembryonic hybrids are both useful for future breeding, but those with apomixis have more immediate commercial application. Consequently, putative apomictic hybrids are propagated in greater numbers (via cuttings) to maximise replication and data precision in rootstock field trials. Use of the MITE marker has enabled maximum replication of putative apomictic hybrids, dramatically reducing the size and cost of field trials and hastened the establishment of seed-source trees. We consider it to be the first useful molecular marker in citrus breeding.