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Wheat Information Service
Number 83: 15-18 (1996)
Maintenance of haploid genome of Agropyron
junccum in wheat
H.C. Sharma
Department of Agronomy, Purdue University, West Lafayette, IN
47907, U.S.A.
Summary
Gametocidal chromosomes have been reported in wide crosses in wheat
(Triticum aestivum, 2n=AABBDD=42), and asymmetric genome
reduction is known to occur in polyploids. Here I report on the
maintenance of 7 univalents, like a gametocidal genome, of
Agropyron junccum (syn. Thinopyrum bessarabicum,
2n=JuJu=14) which resisted elimination for several generations of
selfing of the backcross-1 (BC 1 ) of a wheat x A.
junccum hybrid. The BC1 plant had 49 chromosomes. The
chromosome number in BC1F2 varied from 45 to
52. Of the 12 BC1F3 seeds analyzed from a
49-chromosome BC 1 F2 plant, 9 had 49 and 3 had
49+t chromosomes. Among the 28 BC1F4 seeds
studied from a 49-chromosome BC1F 3 plant, only
one plant had 46+2t while the rest all had 47+t - 51 chromosomes.
When BC1F5 seeds of the bulk harvest from the
BC1F4 plants having 49+t or 49+2t chromosomes
were scored, they had 48-49+2t chromosomes. The occurrence of plants
with 49 chromosomes up to several generations of selfing shows that
the 7 chromosomes of A. junccum have a selective advantage.
It is likely that these chromosomes are being transmitted through
only female gametes. Even though, the phenomenon of directed movement
of the univalents cannot be ruled out, the study provides an example
where the whole genome from a diploid alien species is retained in
wheat due to preferential transmission like individual gametocidal
chromosomes.
Introduction
Gametocidal chromosomes have been reported in wide crosses of
wheat (Triticum aestivam) with alien species. These
chromosomes are preferentially transmitted and maintained from
generation to generation. Such chromosomes have been reported in
various species of Aegilops (Mean 1975, Endo 1982, Endo and
Tsunewaki 1975, Miller et al. 1982, Finch et al. 1984, Tsujimoto and
Tsunewaki 1984). A chromosome of Agropyron elongatum
(Kibirige-Sebunya and Knott 1983) and a chromosome segment ofA.
distichum (Marais 1990) have also been reported to be
gametocidal.
Asymmetric genome reduction, rather than a random loss of
chromosomes, is known to
occur in allopolyploids, in hybrids of polyploids with other species
as well as in artificially induced amphiploids (Gottschalk 1971).
Ladizinsky and Fainstein (1978) described a case of genome partition
in a hybrid of hexaploid oat (2n= 42) x wild tetraploid oat (2n=28)
where a backcross plant with 42 chromosomes produced a tiller with 14
chromosomes which showed chromosome pairing of 6-7 bivalents. This
meant that most probably a set of homologous genomes had split to
give rise to this tiller. Dewey (1980) gave examples in grasses where
higher ploidy amphiploids spontaneously stabilized at the octoploid
level indicating loss of some genomes and retention of some others.
Backcross-2 (BC2) data for Agropyron ciliare (2n=28) x wheat
hybrids, and for A. trachycaulum (2n=28) x wheat hybrids
showed a high proportion of 49-chromosome plants having 7-8
univalents (Sharma and Gill 1983a), indicating splitting of genomes
of the Agropyron species. The 49- chromosome plants likely
arose from the fusion of 28 chromosome (21 wheat+7 Agropyron)
female gametes and 21 chromosome (wheat) male gametes. When Tomar et
al. (1995) backcrossed monosomic 5B of wheat x A. junccum
(2n=14) F1 to wheat, the chromosome number among BC2
plants varied from 38 to 48, and the mean number of univalents in a
48-chromosome plant studied was 7. This chromosome number was
attributed to meiotic non-reduction in the hybrid and backcrosses.
Likewise, partial amphiploids from wheat x segmental polyploid
Agropyron species backcrosses have been recovered due to fusion
of unreduced gametes of the hybrids with wheat gamete (see Sharma et
al. 1987).
Perpetuation of 7. univalent chromosomes, like a gametocidal genome,
of A. junccum; which resisted elimination for several
generations of selfing of the BC1 of a wheat x A.
junccum hybrid is reported.
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