زنبور عسل(ژنتیک -مقالات -پرورش-بیماریها- تغذیه -تازها-و....)
Genetics of the Honey Bee
The family relationships within a colony of bees are different from other agriculturally important animals as a consequence of mating habits, social structure, and drones developing from unfertil ized eggs. The honey bee colony found in nature is a complex family group, best described as a Superfamily. This superfamily consists of: (1) one mother queen, (2) several father drones present as sperm in a sperm storage organ (spermatheca) of the queen, and (3) the worker and drone offspring of the mother and fathers.

Within a superfamily are usually 7 to 10 subfamilies, that is a group of workers fathered by the same drone. Since all the sperm produced by a drone are genetically identical, each subfamily is composed of sisters that are more closely related than full sisters of other animals. Thus, workers belonging to the same subfamily, often called supersisters, have three-quarters of their genes in common by descent. They receive identical gametes from their father and, on the average, half-identical gametes from their mother.

Workers belonging to different subfamilies have the same mother but different fathers. They are half sisters and have one-quarter of their genes in common by descent. On occasion, brother drones mate with the same queen. In such instances, their subfamilies are related to each other as full sisters rather than half sisters. Through natural mating, such full sisters probably are uncommon.

Despite the complicated family structure, the basic principles of genetics still apply to bees. The chromosomes contain hereditary units called genes. The specific place on a chromosome where particular genes are found is called a locus. On rare occasions, a gene entering an egg or sperm has changed somewhat and will have a different effect than the original gene. The process of change is called mutation, and all the forms of a gene that might occur at a locus are called alleles.

Honey bee eggs hatch regardless of whether the are fertilized. The female bees--queens an workers--develop from fertilized eggs that contain 32 chromosomes. These 32 chromosomes consist of two sets of 16, one set from each parent. Hence female bees are said to be diploid in origin. The males (drones) develop from unfertilized egg which contain only one set of 16 chromosomes from their mother. Drones are thus haploid in origin This reproduction by the development of unfertilized eggs is called parthenogenesis

Since queens and workers have paired chromosomes, they carry two alleles for each gene, one on each member of the pair. If both alleles are of the same type, the condition is homozygous; if they are different, the condition is heterozygous. In some heterozygous circumstances, one allele will mask the expression of the other and is said to be dominant. The allele which has its expression masked is said to be recessive. Drones can carry only one type of allele because they are haploid; thus, they are called hemizygous.

At one time parthenogenesis was considered to be the basis of sex determination in bees. The theory was that a chromosome dosage effect occurred such that two sets of chromosomes resulted in females and one set resulted in males. While this is a reasonable explanation, this theory is now known to be untrue.

Research workers investigating apparent low egg viability in inbred lines discovered that sex in bees is determined by the alleles at a single locus. If an egg is a heterozygote at this locus, it will develop into a female. If it is homozygous or hemizygous, it will develop into a male.

The apparent nonviable eggs found in the inbred lines were diploid eggs homozygous at the sex locus. Worker bees selectively remove and destroy homozygous diploid larvae from the comb just after they hatch. Research efforts have been made to rear these diploid drones to maturity with the hope of producing diploid sperm and triploid queens and workers. However, artificial rearing is a difficult procedure; and the resulting diploid drones had reduced testes and produced very little sperm.

While sex determination is genetically complicated, other characteristics can be even more complicated. Different combinations of alleles at a locus result in different expressions of charac teristics. Alleles at other loci also can affect a characteristic. All these different events result in complex genetic systems which produce a wide variety of character expression in bees. For ex ample, worker bee response to isopental acetate (a component of alarm pheromone) was estimated to be influenced by at least seven to eight genes. This variety is some of the raw material necessary for the genetic improvement of bee stocks.

As with other animals, variety in bees is further increased by events that occur when a female (queen) produces an egg. During this time pairs of chromosomes in cells that are destined to be come eggs exchange segments. Further in the process, the chromosome number of germinal eggs is halved. This process results in a haploid egg, with chromosomes having a new combination of alleles at the various loci.

Unfertilized, the egg will develop into a drone which will produce sperm. The processes of re-combination of alleles and reduction of chromosome number do not occur in drones. All the sperm cells produced by a drone are genetically identical. They are identical to each other, and they are identical to the chromosomes in the unfertilized egg that developed into the drone

+ نوشته شده در  چهارشنبه هشتم خرداد 1387ساعت 17:40  توسط مجتبی رحیمی  |