232
chromosomal rearrangements, and the distal region of chromosome arm 6U#1L is
actually homoeologous to the distal region of group-1L chromosomes (Zhang et al.
1998 ; Danilova et al. 2014 ). Because the missing 6BL segment in the Transfer
translocation is replaced by a nonhomoeologous group-1 long arm segment, which
causes duplications/defi ciencies, the resulting Transfer translocation is a noncom-
pensating type. Because radiation tre atment induces chromosome breaks at random,
most of the resulting translocations are betwe en nonhomoeologous chromosomes and
are noncompensating. Stringent selection among noncompensating translocations
needs to be used to recover transfers with acceptable agronomic performance.
9.11 Ae. comosa
Ae comosa Sm. in Sibth. & Sm. (2 n = 2× = MM) is the so urce of the stripe rust resis-
tance gene Yr8 and the stem rust resistance gene Sr34 that were transferred to wheat
by induced homoeologous recombination using a high-pairing line of Ae. speltoides
(Riley et al. 1968a , b ). The wheat– Ae. comosa recombinant chromosome in the
genotype Compair consists of a distal segment of chromosome 2DS, most of the
long arm of 2M#1, and the complete short arm of 2M#1, and it was designated
T2DS-2M#1L•2M#1S. Miller and coworkers (Miller et al. 1988 ) produced inde-
pendent 2D/2M#3/8 and 2A/2M#4/2 Yr8 / Sr34 transfers. The 2D/2M#3/8 transfer is
similar to the wheat– Ae. comosa recombinant chromosome present in Compair, and
the 2A/2M#4/2 recom binant chromosome was identifi ed as T2AS-2M#1L•2M#1S
(Nasuda et al. 1998 ). The structures of these wheat– Ae. comosa recombinant chro-
mosomes suggested that chromosome 2M#1 was rearranged and that the distal
region of the long arm was actually derived from 2M#1S. Molecular marker analy-
sis confi rmed this rearrangement and revealed that both translocations 2D/2M and
2A/2M are noncompensating (Nasuda et al. 1998 ). The advantage of using induced
homoeologous recombination to transfer an alien gene to wheat is that usually
recombination events involve homoeologous chromosome regions and, therefore,
are more likely to be agronomically useful, compensating type. However, this is
only the case in which the wheat and alien chromosomes are not structurally rear-
ranged. Any rearrangement, as in the case of chromosome 2M#1 and most cereal
rye chromosomes, will result in noncom pe nsating transfers. This is the most likely
reason that the Yr8 / Sr34 transfer was not successfully used in wheat improvement.
9.12 Ae. peregrina
Ae. peregrina (Hack. in J. Fraser) Maire & Weller (2 n = 4× = 28, U p U p S p S p ) is the
source of the leaf rust resistance gene Lr59 that was transferred to wheat by Marais
et al. ( 2008 ). The Lr59 transfer occurred spontaneously and monosomic, meiotic
pairing, and microsatellite analyses suggested that the translocation present in
P. Zhang et al.