The term balanced tertiary trisomic has three words of which (1) “trisomic” indicates the presence of extra chromosome, (2) “tertiary” indicates that the extra chromosome is a trans-located chromosome, and (3) “balanced” refers to the breeding behaviour of the trisomic. Ramage defined the BTT as a tertiary trisomic constructed in such a way that the dominant allele of a marker gene, closely linked with the translocation breakpoint of the extra chromosome is carried on the extra chromosome, and the recessive allele is carried on the two normal chromosomes that constitute the diploid complement. The dominant marker gene may be located on the centromere segment or the trans-located segment of the extra chromosome.

1. BALANCED TERTIARY TRISOMICS
IN HYBRID SEED PRODUCTION
GP-604 Cellular and Chromosomal
Manipulations in Crop Improvement
Speaker:
B.Rachana
RAD/2018-18
Ph.D 1st year
(GPBR)

2. BALANCED TERTIARY TRISOMIC
The term balanced tertiary trisomic has three
words of which
(1) “balanced” refers to the breeding behaviour of
the trisomic
(2) “tertiary” indicates that the extra chromosome
is a trans-located chromosome and
(3) “trisomic” indicates the presence of extra
chromosome.

3.  Ramage defined the BTT as a tertiary trisomic
constructed in such a way that the dominant allele
of a marker gene, closely linked with the
translocation breakpoint of the extra chromosome
is carried on the extra chromosome and the
recessive allele is carried on the two normal
chromosomes that constitute the diploid
complement.
 The dominant marker gene may be located on the
centromere segment or the trans-located segment
of the extra chromosome.

5. BREEDING BEHAVIOUR OF BTT
BTT produces the following 3 types of functional
gametes:
(i) ‘n’ Type which consists of both the normal
chromosomes and is functional on both male and
female sides
(ii) ‘n + 1 type consisting of 2 normal and one
trans-located chromosomes, functional on female
side only
(iii) ‘n + 1’ type consisting of normal
chromosomes

7.  Ramage (1965) suggested a method for the use of tertiary
trisomics to supply a male sterile source to be used as
female parent for hybrid seed production.
 In general for hybrid seed production, the ♀ parent should
be either hand emasculated or should be ♂ sterile to elimate
the possibility of selfing.
 Hand emasculation may not be feasible at a commercial
scale and there fore, a male sterile source is always
welcome.
 Further male sterile line has to be maintained for future use.
In case of cytoplasmic male sterility – restoration system,
we use 3 lines
BTT in Hybrid seed production

8.  Male sterile line to be used as ♀ parent (A)
 Restorer line to be used as ♂ parent (R)
 A maintainer line to be used as pollen source for
maintenance of ♂ sterile line.
 When A and R line are crossed the F1 hybrid is
fertile due to restorer gene and there fore can be
used for commercial cultivation, but the F1
hybrid between A x B (lacking of restorer gene)
is male sterile and there fore can be used as ♀
(male sterile) parent again for hybrid seed
production.

9.  Balanced tertiary trisomic is utilized for the
production of male sterile female parent to be used
in hybrid seed production.
 The BTT is constructed in such a way that the gene
for male sterility (ms) is located on the normal
chromosome, while its dominant allele (Ms) is
located on the extra trans-located chromosome.
 In this case, the selfed progeny will consist of BTT
which carries the Ms gene and are male fertile,
while the diploids will carry gene for male sterility
(ms).

10.  Selfed progeny of a BTT will produce 3 types of
plants : diploid, primary trisomic and BTT.
 The BTT will be identified by the presence of
the dominant marker.
 Transmission of the extra chromosome is nil
through pollen, while through the egg, it is
transmitted, but with a low rate.
 The progeny of BTT consists of 30% BTT and
70% diploid, and a very low frequency of
primary trisomics.
 Diploid and the primary trisomics in the progeny
possess recessive alleles of the marker gene.

12. PRODUCTION OF HYBRID SEED:
 Crossing block consists of alternate strips of female
and male rows. The male parent (pollinator) is
normal diploid, a commercial cultivar, whereas the
female parent is the diploid progeny of BTT.
 Trisomics are weaker and late flowering and they
may be rouged to produce pure stand of male sterile
diploids.
 In barley, seeds at a rate of 25-30 kg hectare are
sown. At this seed rate, the trisomic plants are
almost completely eliminated by competition. The
seeds produced in female rows are hybrid seeds.

14. Two types of fields are used for commercial seed
production.
 Trisomic seed production field, where the seed
harvested from trisomic plants will consist of
trisomics and diploids.
 A hybrid seed production field, in which female
rows will be sown using a part of the seed
harvested from the trisomic seed production field
and male rows will be sown with the seed of a
desirable inbred line having good combining
ability with the ♀ male sterile line.

15. MAINTENANCE AND PRODUCTION OF BTT:
There are different strategies for the derivation of ♀
parent from a BTT for the hybrid seed production field
have been suggested.
 In this scheme, seed harvested from BTTs is
sown in the trisomic seed production field at the
rate of 5-7 kg / ha.
 After 3-6 weeks, the diploid seedlings are
roughed out using the information gene(s).

16.  Thus leaving a pure strand of trisomic plants.
 The seed harvested from these trisomic plants is
divided into two parts.
 One part to be used again in the next cycle of
triosmic seed production as above, and
remaining part is used as the female rows in the
hybrid seed production field, where the trisomic
plants (30%) are crowded out and the diploid
plants give hybrid seed.

17.  In the second scheme, the diploid seedlings
are not rogued out and the seed from the
trisomic and diploid plants are harvested
separately using the informational genes.
 The seed harvested from trisomic plants is
recycled in the trisomic seed production
plot, the seed from diploid plants is used for
planting ♀ rows (male sterile) of the hybrid
seed production crossing block.

18.  In the third scheme, the trisomic seed production field is
used as a crossing block with male and female rows.
 The seed harvested from BTTs is used to plant male
rows and the seed harvested from diploid sibs (male
sterile) is used to plant ♀ rows.
 The diploid seedlings from the male rows are rogued
out leaving a pure stand of trisomic plants in the male
rows.
 In this scheme, the seed harvested from ♀ rows of
trisomic seed production field is divided in to two parts,
of which one is recycled to plant female rows in the
trisomic seed production field and the other is used to
plant female rows in the hybrid seed production plot.

19.  A large number of BTTs have been produced in
barley possessing the male sterile genes such
asmsg 1, msg 4,msg 6 and rn.sg 24 etc.
 In this crop, the first hybrid variety named
“Hembar” was produced in U.S.A. by Ramage
and Wiebe in 1969. Because of problems of
cross fertilization, susceptibility to infection
by ergot, and the complexity of the system,
the BTT method has not proved practically
for implementing hybrid barley on a
commercial scale.