107varied over a very wide (0.0–77.8 %) range. The lack or very low level of crossabil-
ity between certain rye types and a particular tetraploid wheat suggest the occur-
rence of a mutation signifi cantly inhibiting crossability at the tetraploid level in the
evolution of wheat. This conclusion contradicted the earlier fi ndings of Riley and
Chapman ( 1967 ), who suggested “that the fi rst hexaploid wheats were probably
able to hybridize fully with rye.” The crossability of tetraploid T. timopheevii with
rye was also studied by Mujeeb-Kazi ( 1981 ).
Various rye species were studied as po llinators in wheat-rye crosses by Kiss
( 1968 ). The highest average seed set on a large number of wheat genotypes was
achieved with Secale vavilovi (18.5 %) and the lowest with Secale fragile (0.4 %),
while the seed set with S. montanum , S. cereale and S. africanum was intermediate
over the course of several years. Halloran ( 1981 ) also studied the effect of r ye geno-
types (various Secale species and S. cereale cultivars) on crossability with tetraploid
wheats. The best seed set was achieved with a Secale vavilovi line, as reported by
Kiss ( 1968 ). Various rye ( S. cereale ) cultivars resulted in different levels of seed set.
The species Secale segetale , S. anatolicum , and S. montanum were also included in
the experiment. Variation in the crossability l evel was found to be due to genetic
variation, in both tetraploid wheat and rye (Halloran 1981 ). Zeven and van Heemert
( 1970 ) concluded that there was a strong correlation between the crossability of
wheat with rye ( S. cereale ) and weed rye ( S. segetale ). The same crossability barrier
acted in wheat × rye and in wheat × weed rye crosses. Five inbred lines of rye ( S.
cereale ) and an open-pollinated rye cultivar were used by Scoles ( 1983 ) to pollinate
wheat cultivars with differing crossability. No hybrid seed was produced using a
cultivar with lo w crossability, while a highly crossable culti var gave an average
seed set of 65 %. Signifi cant differences were detected between the inbreds in terms
of seed set and the weight of F 1 seed. The signifi cant effect of the rye genotype on
crossability in crosses between wheat and rye was also observed by Tanner and Falk
( 1981 ) and Oettler ( 1982 ).
4.2 Mechanism of Pollen Tube Growth Inhibition in Non-
crossable Wheat Varieties ( Kr Alleles) When Pollinated
with RyeZeven and van Heemert ( 1970 ) pollinated ears of 28 wheat varieties having different
crossability using the pollen of weed rye. No crossing barrier was observed in the
stigma, style or ovary wall, because pollen tubes of weed rye were seen in these tis-
sues irrespective of the wheat variety used as female parent. The pollen grains ger-
minated within 6 min after pollination and the tubes reached the region of the
micropyle after about 40 min. Similar observations were made by Lange and
Wojciechowska ( 1976 ), who studied the action of the crossability genes of wheat by
crossing the readily crossable wheat cv. Chinese Spring, the poorly crossable Hope
and the disomic substitution CS/Hope 5B with rye. A comparison of crossability
4 The Crossability of Wheat with Rye and Other Related Species