ROLE OF B CHROMOSOME IN PLANTS

SEMINAR

Speaker:Anshuman TiwariM.Sc.(Ag) 2nd Yr. 3rd Sem.

Dept of Genetics & Plant Breeding

What is B Chromosome?

These are particular kind of chromosome that may or may not be found in an organism as extra chromosome over and above the standard diploid or polyploid chromosome complement.

Also known as accessory or Supernumerary chromosome.

HISTORY E. B. Wilson(1907): First discovered in leaf

footed plant bug i.e. Metapodius.

1920: It come to notice in rye.

Gotoh(1924): Clearly define supernumerary nature.

Kuwada(1925): Reported in Maize.

Randolph(1927):Classified them as a B chromosome.

E. B. Wilson

Metapodius

GENERAL PROPERTIES

Smaller in size & carry less DNA than A chromosome.

Non mendelian inheritance i.e. No pairing & random distribution during cell division.

Carry no genes with major effects.

More tolerant to deletion & other structural changes.

Differ morphologically from A chromosome

OCCURRENCE & DISTRIBUTION IN PLANTS

Darlington,1956: Bs are mainly restricted to diploid species than polyploid.

But in some species like Leucanthemum, Agrostis flaccida etc. Bs were found only in tetraploid species.

Bs mainly confined to outbreeding species(Moss, 1969).

Muntzing,1954 & Moss,1969 demonstrated that enforced inbreeding leads to decline in Bs frequency.

Source: Jones R. N. & Rees H. (1982). Distribution of Bs among species. B Chromosomes,1:11-14

OCCURRENCE & DISTRIBUTION IN PLANTS

979 sp are carrier of B chromosome

40 woody species

8% of Monocoat3% of Dicot

Compositeae – 270 Sp.Gramineae – 266 Sp.Liliaceae – 189 Sp.

Survey of 23,652

angiosperm species

(Levin et al, 2005)

EFFECTS

Slight & difficult to detect

Usually additive

Odd & even effect

Control chromosome pairing in

hybrid

Both favorable & unfavorable

to plants

EFFECTS OF Bs ON PLANTS

1. EFFECTS ON GROWTH & DEVELOPMENT

S.NO. CHARACTERS EFFECT SPECIES

1. Germination Delayed Speed up

Secale cerealeAllium porrum

2. Growth & vigourReduced

Aegilops speltoidesZea MaysSecale cereale

3. Flowering time Delayed

Secale cerealeZea MaysAllium spp.

4. Fertility Reduced

Aegilops speltoidesZea maysSecale cereale

5. Seed weight Increased Secale cereale

Ref: Randolph(1941), Moss(1966), Vosa(1966), Mendelson & Zohary(1972)

Source: Masonbrink R.(2012),An Investigation of Maize B Chromosome(Dissertation)

Maize plant with 13 B chromosome

Neil et al (2008)

Effect of Bs on vigour (rye plant)

Effect of Bs on pollen grain viability (Aegilops speltoides)

Kour et al (2012) Observe no significant change in presence of Bs in morphology.

Presence of 1 or 2 B chromosome did not cause major reduction in fertility.

But further increase in Bs reduces fertility.

Sometime plant become sterile in presence of more than 2Bs.

Plantago lagopusSource:Kour et al, 2012. Analysis of morphological effects of Bs in Plantago,, 2(4): 598-602.

2. EFFECT ON CELL SIZE & MASS

Muntzing & Akdik(1948): study the length of stomatal guard cells of leaves of rye plant & first to confirm that cell size increased with increasing B frequency.

The figure shows the effect of Bs on fresh weight of meristemetic cells & nuclear DNA content, which is closely correlated to volume in root tip of rye plants with from 0B to 6B.(John & Jones,1970).

Source: Jones R. N. & Rees H. (1982).Effects of B chromosome in plants. B Chromosomes,1:75-82

3. CHROMOSOME BEHAVIOUR AT MITOSIS

1. KNOBS & B CHROMOSOME IN MAIZE:

Rhoades & Dempsey(1972) have shown that maize chromosome with knobs & the presence of B chromosome give rise to highly unusual chromosome behaviour.

Both phenomenon are due to failure of separation at heterochromatic knob during anaphase of 2nd pollen mitosis.

They conclude that both chromosome have common organization because nondisjuction is controlled by same element.

PROPHASE

ANAPHASE

SPERMCELL

Deficient Normal 2B 0B

KNOBED A CHROMOSOME B CHROMOSOME

DIAGRAM TO DRAW ATTENTION TO THE PARALLEL BETWEEN THE MECHANISM OF KNOBBED A CHROMOSOME & B CHROMOSOME

Source: Jones R. N. & Rees H. (1982).Effects of B chromosome in plants. B Chromosomes,1:11-14

Genetic organization and nondisjunction property of the rye B chromosome

Ref: Jones R. N., Viegas W., Houben A.(2008), Annals of Botany 101: 767–775.

2. Secale cereale:

3. Triticum grandiflorum:

B Chromosome have been shown to enhance the rate of spontaneous breakage in A chromosome.

Breakages are tissue specific & occur extensively in endosperm.

Rutishauser (1956): Shown that in Triticum grandiflorum the seed fertility is influenced by the introduction of up to three euchromatic Bs in the endosperm.

4. CHROMOSOME BEHAVIOUR AT MEIOSIS

Darlington(1956) was first to suggest that B chromosome might influence the amount of genetic variability within populations.

It have some control over the amount & distribution of crossing over within A chromosome at meiosis.

Moss(1966) showed that the variability within progenies of rye plants carrying Bs was greater than within plant without Bs.

No of Bs in seeds

Seeds from 0B plants Seeds from 2B plants

Germination time

Seedling Height(cm)

Germination time

Seedling Height(cm)

0 0.33 11.59 0.57 16.25

2 0.62 10.95 8.88 16.25

4 - - 4.12 32.15

Ref: Jones R. N. & Rees H. (1982).B Chromosomes . Effects, 4: 69-110

1. Frequency & distribution of chiasmata at meiosis:

SECALE CEREALE:

B chromosome show effect on the distribution of chiasmata both between PMC within anthers & between bivalents within PMC.

The range of chiasma frequencies in 0B & 8B rye plant are given below(Jones & Rees, 1967):

No of Bs Chiasma frequencies/bivalent

Chiasma frequencies/cell

0B 1 to 3 12 to 16

8B 0 to 4 6 to 18

Infuence of B chromosome on chiasma frequencies in three species i.e. Maize, Lolium & Listera.

Ref: Jones R. N. & Rees H. (1982).B Chromosomes. Effects, 4: 69-110

2. The control of chromosome pairing in species hybrid:

a) Lolium temulentum × L. Perenne

Evans & Macefield(1972,1973) investigated meiosis in F1 hybrid which carry 2B chromosome of L. Perenne.

B chromosome reduces the no. of bivalents in hybrid.

Thus , depress the amount of effective pairing & chiasma formation at meiosis.

Design of experiment by Evans & Devis to demonstrate suppression of

homeologus pairing by B chromosome

Metaphase-I of meiosis in Lolium sp hybrid

Ref: Jones R. N. & Rees H. (1982).B Chromosomes 4,100-102

b) Chromosome pairing in Wheat hybrid

Hexaploid Wheat(2n = 6x = 42)

AA BB DD

Riley and Chapman (1958);The activity of 5B chromosome is similar to Bs.

In certain wheat hybrid Bs are able to compensate for missing 5B.

What control chromosome pairing in wheat hybrid ?

Riley & Sear (1954) observe that deleting chromosome 5B in wheat induced crossover between homeologues.

Ph1 locus in 5B was the major regulator of recombination.

Ph1 action is to promote homologues to pair.

Ref:Riley, R and Sears, E. R. (1954) Genetic control of the cytogenetically diploid behaviour of hexaploid wheat. Nature 182:713–715.

The chromosome 5B can be an efficient way to induce homoeologous pairing between chromosomes of wheat and Aegilops species for precise introgression of useful variability.

Triticum aestivum x Aegilops sp.

Chromosome pairing in T. aestivum/Ae. Peregrine F1 hybrids with and without

5B chromosome

Gross morphology of plants and spikes of T. aestivum/Ae. peregrina interspecific

hybrids with and without 5B chromosome.

Ref:Imran, S. et al (2015). Characterization of interspecific hybrids of Triticum aestivum x Aegilops sp.

CONCLUSION

B chromosome are not essential for normal growth & development, there effect upon phenotype are manifold, often pronounced & startling.

They affect cell size, duration of cell division, protein & RNA content of cells, distribution of chiasmata & chromosome pairing in species hybrid at meiosis.

Many of the effects of Bs are deleterious to fitness but there effect on crossing over at meiosis could have adaptive significance in generating novel & superior genotype.

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