Balancer chromosome
Encyclopedia
Balancer Chromosomes are special, modified chromosomes used for genetically screening a population of organisms to select for heterozygotes. A balancer chromosomes can be used as a genetic tool used to prevent crossing over (genetic recombination
) between homologous chromosome
s during meiosis
. Balancers are most often used in Drosophila melanogaster
(fruit fly) genetics to allow populations of flies carrying heterozygous mutations to be maintained without constantly screening for the mutations but can also be used in mice . Balancer chromosomes have three important properties: they suppress recombination with their homologs, carry dominant markers and negatively affect reproductive fitness when carried homozygously.
. At this point in time scientists had already realized the benefits of being able to genetically screen populations of organisms for genetic study. Equally as important, they also realized that they could limit crossing over
in these populations as well give them a very consistent genetic population.
The use of balancer chromosomes has evolved into a well known and widely used method for genetic screening
of model organisms. They are even being used to investigate the role of heterochromatin packing and the effect it has on genes as well as studies on the effect telemeres have on gene silencing.
, balancer chromosomes are the products of multiple, nested chromosomal inversion
s so that synapsis
between homologous chromosomes is disrupted. This construct is called a crossover suppressor. If crossing over between a balancer chromosome and the balancer's homolog does occur during meiosis each chromatid
ends up lacking some genes and carrying two copies of other genes. Recombination in inverted regions leads to dicentric or acentric chromosomes (chromosomes with two centromeres or no centromere). Progeny
carrying chromosomes that are the products of recombination between balancer and normal chromosomes are not viable (they die).
Dominant markers such as genes for green fluorescent protein
or enzymes that make pigments allow researchers to easily recognize flies that carry the balancer chromosome. By suppressing reproductive fitness when carried homozygously a balancer chromosome ensures that the population it is carried in does not become fixed for the balancer chromosome.
Balancer chromosomes always contain a lethal recessive allele. This means that if an organism receives two copies of the balancer chromosome, one from the mother and one from the father, then the organism will not live. So any organism that is Homozygous for that chromosome will not live to pass on its genes. However, offspring that only get one copy of one balancer chromosome and one copy of a wild type or mutant chromosome will live to pass on its genes. After only a few generations the population will be entirely heterozygous so that you can be guaranteed of its genotype on at least those two chromosomes.
Balancer chromosomes also come with some sort of physical marker. This marker can be actually associated with the DNA
in the chromosome such as the Green Florescent Protein that fluoresces in ultraviolet light, or it can be an easily distinguishable physical characteristic. These physical characteristics can be anything that is seen easily. In Drosophila melanogaster for example, eye color and hair length are commonly used. This physical marker serves as a double check that you indeed have the heterozygous balancer chromosomes in the organism.
the balancer carries. The naming of balancer chromosomes has been standardized as follows: the first letter of the chromosome's name represents the number of the chromosome it stabilizes. F stands for the first chromosome, S stands for second, and T stands for third. (The small fourth chromosome does not undergo recombination and therefore does not require balancing.) This letter is then followed by M, for "multiply inverted". The M is followed by a number to distinguish balancers of the same chromosome. Additionally, a list of genetic marker or markers of the balancer are listed after the name and separated by a comma. Generally mutations with easily observable dominant phenotypic traits that are often homozygous lethal are used to ensure that all progeny are heterozygous. For example, the commonly used "TM3, Sb" balancer is a balancer chromosome that stabilizes the third chromosome and carries a mutant Sb (Stubble) gene as a marker. All flies containing the TM3, Sb balancer will have shortened (or stubbly) hairs on the back of the fly, which are easily seen when viewed through a microscope. The "3" is to distinguish this from other third chromosome balancers, such as TM1 and TM2.
A line is said to be "double-balanced" if it is heterozygous for two different balancer chromosomes (i.e. TM6,Tb/TM3,Ser) on a balancer chromosome and a homozygous lethal, heterozygous visible mutant on the other, wild-type chromosome (i.e. D/TM3,Ser). Most balancer chromosomes also carry a recessive allele such as the ebony mutation
that is only manifest in these stocks with two balancer chromosomes. These stocks are often used to provide sources of easily traceable traits when breeding two different lines together so that the correct progeny of each cross might be selected. Stocks double-balanced at both the 2nd and 3rd chromosome in Drosophila are widely available.
They achieved this by using a gene isolated from yeast. This gene is called FLP recombinase and causes large chromsomal inversions. Through trial and error they found that they could recombine the chromosomes such that each has the recessive mutation while the other half contained half of a balancer chromosome with a physical marker and a lethal recessive. The other homolog did not contain the lethal recessive in the lines that survived. Figure 1 in the paper illustrates the screen. This new technique allowed recessive screening in 95% of the Drosophila genome. It also greatly improved yields in germ line mutations.
Another published paper that employed the use of balancer chromosomes is | "Inhibition of RNA Interference and Modulation of Transposable Element Expression by Cell Death in Drosophila." This paper should be discussed, because it demonstrates the power of balancer chromosomes and what you can accomplish with genetically stable lines. A line was established that exhibited low levels of cell death and was named EGFPir hs-hid. The RNAi levels were analyzed and they found interesting results in the cells undergoing low levels of cell death and the surrounding cells in the tissue. They found that these cells would shut down their RNAi mechanism via maintaining RNA in a double stranded state. If RNA remains in a double stranded state then the RNAi mechanism of gene silencing is shut down.
They speculated that this response was an evolutionary trend toward redundant immune response against RNA viruses. If one cell is already undergoing cell death to attempt to stop spread of a virus, then the RNAi immune response has been ineffective. This causes another immune response that attempts to stop the virus which is binding double stranded RNA and keeping double stranded so that it cannot be transcribed into virus proteins. The mechanism of maintaining double stranded RNA is not known.
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
) between homologous chromosome
Homologous chromosome
Homologous chromosomes are chromosome pairs of approximately the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father...
s during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. Balancers are most often used in Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
(fruit fly) genetics to allow populations of flies carrying heterozygous mutations to be maintained without constantly screening for the mutations but can also be used in mice . Balancer chromosomes have three important properties: they suppress recombination with their homologs, carry dominant markers and negatively affect reproductive fitness when carried homozygously.
History of Balancer Chromosomes
Balancer Chromosomes were used as far back as 1975 in the paper | Analysis of Chromosome 4 in Drosophila Melanogaster11: Ethyl Methanesulfonate Induced Lethals. Random mutations are caused by feeding the larva ethyl methane sulphonate. When a phenotype of interest is observed, the line is crossed with another line containing balancer chromosomes to maintain their lineage. In one instance they were used to genetically screen a population of Caenorhabditis elegansCaenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...
. At this point in time scientists had already realized the benefits of being able to genetically screen populations of organisms for genetic study. Equally as important, they also realized that they could limit crossing over
Crossing Over
Crossing Over may refer to:* Chromosomal crossover, a cellular process* Crossing Over , a 1998 album by Hesperus* Crossing Over, a book by John Edward* Crossing Over , a 2009 film...
in these populations as well give them a very consistent genetic population.
The use of balancer chromosomes has evolved into a well known and widely used method for genetic screening
Genetic screen
A genetic screen is a procedure or test to identify and select individuals who possess a phenotype of interest. A genetic screen for new genes is often referred to as forward genetics as opposed to reverse genetics, the term for identifying mutant alleles in genes that are already known...
of model organisms. They are even being used to investigate the role of heterochromatin packing and the effect it has on genes as well as studies on the effect telemeres have on gene silencing.
How balancer chromosomes work
To suppress crossing overChromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
, balancer chromosomes are the products of multiple, nested chromosomal inversion
Chromosomal inversion
An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.Paracentric inversions do not include the...
s so that synapsis
Synapsis
Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are...
between homologous chromosomes is disrupted. This construct is called a crossover suppressor. If crossing over between a balancer chromosome and the balancer's homolog does occur during meiosis each chromatid
Chromatid
A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division . They are called sister chromatids so long as they are joined by the centromeres...
ends up lacking some genes and carrying two copies of other genes. Recombination in inverted regions leads to dicentric or acentric chromosomes (chromosomes with two centromeres or no centromere). Progeny
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
carrying chromosomes that are the products of recombination between balancer and normal chromosomes are not viable (they die).
Dominant markers such as genes for green fluorescent protein
Green fluorescent protein
The green fluorescent protein is a protein composed of 238 amino acid residues that exhibits bright green fluorescence when exposed to blue light. Although many other marine organisms have similar green fluorescent proteins, GFP traditionally refers to the protein first isolated from the...
or enzymes that make pigments allow researchers to easily recognize flies that carry the balancer chromosome. By suppressing reproductive fitness when carried homozygously a balancer chromosome ensures that the population it is carried in does not become fixed for the balancer chromosome.
Balancer chromosomes always contain a lethal recessive allele. This means that if an organism receives two copies of the balancer chromosome, one from the mother and one from the father, then the organism will not live. So any organism that is Homozygous for that chromosome will not live to pass on its genes. However, offspring that only get one copy of one balancer chromosome and one copy of a wild type or mutant chromosome will live to pass on its genes. After only a few generations the population will be entirely heterozygous so that you can be guaranteed of its genotype on at least those two chromosomes.
Balancer chromosomes also come with some sort of physical marker. This marker can be actually associated with the DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
in the chromosome such as the Green Florescent Protein that fluoresces in ultraviolet light, or it can be an easily distinguishable physical characteristic. These physical characteristics can be anything that is seen easily. In Drosophila melanogaster for example, eye color and hair length are commonly used. This physical marker serves as a double check that you indeed have the heterozygous balancer chromosomes in the organism.
Naming convention
Balancer chromosomes are named for the chromosome they serve to stabilize and for the phenotypic or genetic markerGenetic marker
A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify cells, individuals or species. It can be described as a variation that can be observed...
the balancer carries. The naming of balancer chromosomes has been standardized as follows: the first letter of the chromosome's name represents the number of the chromosome it stabilizes. F stands for the first chromosome, S stands for second, and T stands for third. (The small fourth chromosome does not undergo recombination and therefore does not require balancing.) This letter is then followed by M, for "multiply inverted". The M is followed by a number to distinguish balancers of the same chromosome. Additionally, a list of genetic marker or markers of the balancer are listed after the name and separated by a comma. Generally mutations with easily observable dominant phenotypic traits that are often homozygous lethal are used to ensure that all progeny are heterozygous. For example, the commonly used "TM3, Sb" balancer is a balancer chromosome that stabilizes the third chromosome and carries a mutant Sb (Stubble) gene as a marker. All flies containing the TM3, Sb balancer will have shortened (or stubbly) hairs on the back of the fly, which are easily seen when viewed through a microscope. The "3" is to distinguish this from other third chromosome balancers, such as TM1 and TM2.
A line is said to be "double-balanced" if it is heterozygous for two different balancer chromosomes (i.e. TM6,Tb/TM3,Ser) on a balancer chromosome and a homozygous lethal, heterozygous visible mutant on the other, wild-type chromosome (i.e. D/TM3,Ser). Most balancer chromosomes also carry a recessive allele such as the ebony mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
that is only manifest in these stocks with two balancer chromosomes. These stocks are often used to provide sources of easily traceable traits when breeding two different lines together so that the correct progeny of each cross might be selected. Stocks double-balanced at both the 2nd and 3rd chromosome in Drosophila are widely available.
Important Scientific Contributions Using Balancer chromosomes
Balancer chromosomes already give geneticists a reliable method for genetically screening organisms for a mutation and keeping that line constant. A new technique using balancer chromosomes is explored in the paper | "The Autosomal Flp-Dfs Technique for Generating Germline Mosaics in Drosophila Melanogaster." This paper showed for the first time that you can screen for a recessive mutation that only shows phenotype when homozygous. Using old balancer chromosome methods, genetic screening only allowed you to pick out heterozygous dominate mutations. This experiment uses clonal screening to detect homozygous individuals and kept them in a constant line.They achieved this by using a gene isolated from yeast. This gene is called FLP recombinase and causes large chromsomal inversions. Through trial and error they found that they could recombine the chromosomes such that each has the recessive mutation while the other half contained half of a balancer chromosome with a physical marker and a lethal recessive. The other homolog did not contain the lethal recessive in the lines that survived. Figure 1 in the paper illustrates the screen. This new technique allowed recessive screening in 95% of the Drosophila genome. It also greatly improved yields in germ line mutations.
Another published paper that employed the use of balancer chromosomes is | "Inhibition of RNA Interference and Modulation of Transposable Element Expression by Cell Death in Drosophila." This paper should be discussed, because it demonstrates the power of balancer chromosomes and what you can accomplish with genetically stable lines. A line was established that exhibited low levels of cell death and was named EGFPir hs-hid. The RNAi levels were analyzed and they found interesting results in the cells undergoing low levels of cell death and the surrounding cells in the tissue. They found that these cells would shut down their RNAi mechanism via maintaining RNA in a double stranded state. If RNA remains in a double stranded state then the RNAi mechanism of gene silencing is shut down.
They speculated that this response was an evolutionary trend toward redundant immune response against RNA viruses. If one cell is already undergoing cell death to attempt to stop spread of a virus, then the RNAi immune response has been ineffective. This causes another immune response that attempts to stop the virus which is binding double stranded RNA and keeping double stranded so that it cannot be transcribed into virus proteins. The mechanism of maintaining double stranded RNA is not known.