1. NAME: EMOKPARE ABASS ABRAHAM
DEPT: PLANT BIOLOGY AND BIOTECHNOLOGY
LEVEL: 400L
MAT NO: LSC0903383
COURSE: CYTOGENETICS (PBB 419)
DATE: 7TH Jan. 2013
B CHROMOSOM1E AND EFFECTS ON PLANT
INTRODUCTION
Bs are dispensable extra chromosomes which are found in only some
individuals of a population, and which are not duplicates of any member of the
basic A chromosome set in diploids or polyploids. They are often
morphologically distinct, usually smaller than the As, and they show
numerical variation within and between individuals. Phenotypic effects are
usually unfavourable, especially with high numbers and inheritance is
irregular and non-Mendelian. We may define B chromosomes as dispensable
supernumeraries which do not recombine with any members of the basic A
chromosome set and which have irregular and non-Mendelian modes of
inheritance. In plants much of the early work on Bs was undertaken in maize,
beginning with Kuwada in 1915, but it was Longley (1927), and later
Randolph (1941) who first distinguished these extras in maize as being
supernumerary and who presented the first detailed study in plants on their
behavior and characteristics. Different names have been used, such as
supernumerary, accessory and extra fragment, but the term B chromosome, or
just B, is now the standard and certainly the most convenient form.
OCCURRENCE
B chromosome is now known in at least 1372 flowering plants, of which 12
are conifers and 1360 are angiosperms. There are also five cases known in
ferns, and a few in the fungi; 738 species of monocots with Bs are known
Jones (1995) , which compares with 510 cited by Jones & Rees in 1982, and
the corresponding numbers for the dicots are 622 compared with 497.

2. ORIGIN
The question of origin has often been asked, and a variety of answers have
been suggested It goes without saying that the Bs originate from the As, and
that there are endless opportunities as errors in crossing over and in spindle
malfunction for fragments of As to be generated. There are numerous
anecdotes and speculations on the origins of Bs, but a shortage of convincing
answers.
PHENOTYPIC EFFECTS
The phenotypic effects of Bs are manifold, and have and have been reviewed
in detail (Jones, 1975; Jones & Rees, 1982). The overview is that low numbers
of them might be neutral but as their frequency increases they become
harmful and lead to reduced viability and especially to loss of fertility. The
effects that have been detected are of quantitative nature and cover all levels
of the phenotype from the whole plant down through the cellular and to the
nuclear and chromosomal level.
Among the most notable observations are the diploidising effects on the A
chromosomes in allopolyploids, the different activity expressed as the odd-
even effect, the interactions between Bs and heterochromatic knobs on A
chromosome recombination in maize, and elimination of chromatin from
knob bearing As. New information covering various aspects of the phenotypic
effect is considered below:
1. Vigour, fertility, germination
Jimenez et al. (1994) established a set of pure breeding lines of rye each
of which was isogenic for its B chromosomes; the entire Bs having
originated from a single 1B plant at the start of the backcrossing
programme. When assessments were made on fertility and vigour
under these conditions it was clear that effects of Bs are expressed
differently in different lines, and that there is interaction between the Bs
and the As of the host genotype. This differential behavior of Bs has
implications for populations: amongst other things it means that Bs will
influence the genetic composition of populations, and their evolutionary

3. behavior, by having more harmful effects on some genotypes than on
others.
In terms of fertility it is worth noting that in rye the average for 2B
plants is of the order of 80% of the 0B values. Contrariwise to the
depression of seed set there are reports that the germination potential
of pollen grains can be enhanced by Bs under in vitro conditions, as
described by Puertas & Carmona (1976) for rye, and more recently by
Jayalakshmi & Pantulu (1984) for pearl millet.
Effects of Bs on seed germination are variable. Bs appear to enhance
germination in Allium porrum, Anthoxanthum alpinum, Picea glauca and
Allium schoenoprasum, whereas their effects are either neutral or
deleterious in some other species.
The significant feature of the chives work is that the laboratory
experiments are directly related to observations made on populations,
and we can have some confidence that the effects are meaningful in
terms of adaptation and the maintenance of Bs in a species which lacks
any form of drive.
A chromosome behavior
B chromosomes affect the behavior of the A set at meiosis. Years of
observations on numerous species of plants have confirmed this fact.
The information up to 1980 is summarized in Jones & Rees (1982), and
specific work on rye is updated in Jones & Puertas (1993). A consistent
feature of virtually all of the studies made to date is that the presence of
Bs affects either the mean cell chiasma frequency of the As, or the
distribution pattern of chiasmata within the A complement or a
combination of both the mean and the variance.
The question as to how Bs influence chiasma formation in the As
remains an open one. One indisputable fact is that each additional B
increases the chromosome complement, and the mass of chromatin, in
the nucleus and some physical and metabolic adjustment must be made.

4. Pairing control in hybrids
The most enigmatic of all known B effects on phenotype is the way that
they can control A chromosome pairing in species hybrids, particularly
their diploidising effect in allopolyploids whereby the metaphase I
associations are between homologous rather than homoeologous
partners. The hybrids mainly concerned are wheat x rye, wheat x
Aegilops spp., species related to hexaploid wheat and Lolium and Festuca
spp. among the forage grasses.
Odd-even effect
Jones & Rees (1967) first alluded to the fact that the Bs in rye could
behave differently according to whether they were present in odd or
even-numbered combinations within the plant. In general, odd numbers
of Bs have a more detrimental effect that even numbers, so that it is
more harmful for a plant to have 1B than it is to have 2Bs. This
observation later extended to maize and to several other species, as
reviewed in Jones & Rees (1982). As it happens the effect was actually
discovered by Darlington & Upcott (1941), who found that maize plants
with odd numbers of Bs have more chiasmata per B than do plants with
even B numbers, although this was not recognized as the odd-even
effect at the time. It might be possible in future to study this aspect with
molecular probes and with confocal microscopy, and to use the Bs as a
model to look at chromosome arrangement and nuclear genetic activity
Jones (1995).
REFERENCE
Darlington CD, Upcott MB. 1941. The activity of inert chromosomes in Zea
mays. Journal of Genetics 41: 275-296.
Jayalakshmi k, Pantulu JV. 1984. Influence of B-chromosomes on pollen

5. grain germination and pollen tube growth in pearl millet. Beitrage zur
Biologie der Pflanzen 60: 255-259.
Jones RN, Puertas, MJ. 1993. The B-chromosomes of rye (Secale cereal L.). In:
Dhir KK, Sareen TS, eds. Frontiers in Plant Science Research. Delhi
(India): Bhagwati Enterprises, 81-112.
Jones RN, Rees H. 1967. Genotypic control of chromosome behavior in rye. XI.
The influence of B-chromosomes on meiosis. Heredity 22: 333-347.
Jones RN, Rees H. 1982. B chromosomes. London: Academic Press.
Longley AE. 1927. Supernumerary chromosomes in Zea mays. Journal of
Agricultural Research 35: 769-784.
Puertas MJ, Carmona R. 1976. Greater ability of pollen tube growth in rye
plants with 2B chromosomes. Theoretical and Applied Genetic 47: 41-43.
Randolph LF. 1941. Genetic characteristics of the B chromosomes in maize.
Genetics 26: 608-631.