B-Chromosomes: Structure, Behaviour and Significance | Botany Optional for UPSC PDF Download

B-Chromosomes

• B-chromosomes are additional chromosomes that exist alongside the standard chromosome set of a species, and they are not crucial for normal growth and development.
• These chromosomes are also known as extra, supernumerary, or accessory chromosomes.
• They were initially identified by E. B. Wilson in the leaffooted plant bug, specifically in the insect Metapodius (now referred to as Acanthocephala) about a century ago. Many of their distinctive characteristics were documented during this early discovery period.
• The term "B-chromosome" was coined by Randalph in 1928.
• These chromosomes share a similar morphology with the regular somatic chromosomes.

History Of B-chromosomes

• In 1907, E. B. Wilson made the first discovery of B-chromosomes in the leaf-footed plant bug known as Metapodius.
  
• In the 1920s, these chromosomes were also observed in rye.
• In 1924, Gotoh provided clear evidence of their supernumerary nature, indicating that they were in addition to the regular chromosome set.
• E. B. Wilson's research contributed to the understanding of B-chromosomes.
• In 1925, Kuwada reported the presence of B-chromosomes in maize.
• In 1927, Randolph classified these additional chromosomes as "B-chromosomes," a term that is still used today to describe them.

General Properties of B-Chromosomes

Here are the general properties of B-chromosomes:

• They have distinct morphological differences compared to A chromosomes.
• B-chromosomes are found in approximately 100 animal species and around 600 plant species, particularly in fungi.
• In maize, they can lead to some reduction in vigor and fertility.
• B-chromosomes are predominantly heterochromatic, meaning they have a high density of inactive genetic material.
• They do not carry genes with major effects on an organism's traits.
• Approximately 90% of B-chromosomes lack a centromere, which is the region responsible for chromosome segregation during cell division. This absence makes them prone to moving toward either the centrioles or poles during cell division.
• B-chromosomes are smaller in size and contain less DNA compared to A chromosomes.
• Their inheritance follows non-Mendelian patterns, meaning they do not undergo typical Mendelian inheritance involving chromosome pairs and random distribution during cell division.

Effects of B-Chromosomes on Plants

The effects of B-chromosomes on plants can vary, and they can have both favorable and unfavorable impacts. These effects are often slight and can be challenging to detect.
Here are some of the observed effects of B-chromosomes on specific plant species:

• Germination:
  • Delayed germination in Secale cereale.
  • Speeds up germination in Allium porrum.
• Growth and Vigor:
  • Increased growth and vigor observed in Aegilops speltoides.
  • Reduced growth and vigor in Zea mays (maize) and Secale cereale (rye).
• Flowering Time:
  • Delayed flowering time in Secale cereale.
  • Delayed flowering time in Zea mays (maize) and Allium spp. (onion family).
• Fertility:
  • Reduced fertility in Aegilops speltoides.
  • Reduced fertility in Zea mays (maize) and Secale cereale (rye).
• Seed Weight:
  • Increased seed weight observed in Secale cereale (rye).

It's important to note that the effects of B-chromosomes can vary among different plant species and even within the same species. Additionally, the mechanisms by which B-chromosomes influence these traits are not fully understood and may involve complex genetic interactions.

Conclusion

In conclusion, B-chromosomes are additional chromosomes that are not essential for the normal growth and development of plants. However, their effects on plant phenotypes are diverse, sometimes significant, and can be quite remarkable. These effects encompass various aspects of cellular and genetic processes, including cell size, the duration of cell division, the content of proteins and RNA within cells, the distribution of chiasmata (points where genetic material is exchanged during meiosis), and chromosome pairing during meiosis.

While many of the effects of B-chromosomes can be detrimental to the fitness of plants, it's important to note that they can also have adaptive significance. One of their notable effects is on crossing over during meiosis, which can lead to the generation of novel and potentially superior genotypes. This suggests that B-chromosomes, despite being non-essential, can play a role in genetic diversity and evolution within plant populations.