Table 3.

Hybridization system concepts for wheat based on non-conditional NGMS.

Strategy	Mutant/Gene	Characteristics	References
First-Generation Concepts
XYZ System	generic	Complementation of a ms mutation with an alien chromosome; three components for female maintenance XYZ; sorting of ms/ms genotype based on vegetative characteristics; 4-line system.	[83]
Modified XYZ System	Cornerstone (ms1c)	Wheat or barley isochromosome proposed for complementation of ms1c; maintenance of Y line through self-pollination, eliminating the need for X component; A x B like cross required for propagation of Z line; 3-line system.	[84]
Second-Generation Concepts
4E-ms	Lanzhou (LZ; ms1g)	Complementation with chromosome 4E from Agropyron elongatum. 4E carries the ms1 homeolog and Blue Aleurone (BA) gene; sorting of ms1/ms1 genotype based on seed phenotype; 2-line system.	[85]
SPT-like System based on recessive sterility	ms1, ms5, Tams26, Tams45	Complementation with a TDNA carrying the wild-type copy of the mutant gene; ms/ms identification based on the seed marker also present in the TDNA; final product is non-transgenic; can be utilized as a 2-line or 3-line system, whichever is efficient.	[27,31,32,33]
SPT-like System based on dominant sterility	TaMS45-pIR	Dominant male sterility; proof of concept transgenic but gene editing can provide a more regulatory-friendly system; manipulation of male lines also needed.	[56]
	DNA (Adenosine-N6-)-Methyltransferase (DAM)	Dominant male sterility; transgenic; manipulation of male lines also needed.	[56]
SPLIT Gene	Barnase (B. amyloliquefaciens)	Based on two components (split) of barnase gene; transgenic; no manipulation of male lines needed; an A x B-like cross needed for maintenance; 3-line system.	[86]
Chemical complementation	TaSpPS	Male sterile Taspps mutants can produce seed with application of oxo-6-octadecane-pyran-2-olate; registration of chemical for commercial use not determined; 2-line system.	[37]
Third-Generation Concepts
MS45-BA	Tams45	Complementation with telosome 4EL from Thinopyrum ponticum; MS1 and BA in tight linkage (same chromosome arm); triple-gene knockouts required for male sterility; 2-line system.	[33,87]
Modified 4E-ms	ms1	Complementation with an engineered alien chromosome; MS1 and BA in tight linkage (same chromosome arm); 2-line system.	[87,88]
Updated XYZ	tip2	Complementation with barley 7H monosomic addition line; seed sorting based on waxy endosperm/seed phenotype; 3-line system.	[14]
MFW-PV-OV Maintainer	generic	Requires complex gene editing or transgenic manipulation to combine MFW, PV and OV components in tight linkage; 3-line system.	[89]