Improving Our Bee Stocks: Why It is So Difficult to Do
Improving Our Bee Stocks: Why It is So Difficult to Do
Writtin by David Yanke in the early 1990’s
The ground I hope to cover with this talk is that mountainous, probably impassable stuff which stands between us and the ever-elusive super bee. The mechanics of heredity are the same whether it be bees or buffalo. The progeny of organisms that reproduce sexually are not exact duplicates of their parents and vary in many ways amongst themselves. This variation is the spice of life. It is what allows a species to adapt and evolve, and also provides plant and animal breeders with a wide array of choices.It is important to understand some of the basics of genetics in order to comprehend much of what is said later. The carrier of the genetic message is DNA, a large molecule which contains the code responsible for the structure and function of any living organism.
Genes are specific lengths of the DNA molecule, the base units of inheritance. Variations of the same gene are called alleles. There may be several different alleles of a gene. Every sexually reproducing species inherits two alleles, one from each parent. These interact with varying degrees on each other.
Chromosomes are cellular bodies found in the nucleus of cells that carry genes. Two sets of genes (made up of many alleles) are carried on a given number of chromosomes characteristic of each species - humans have 46, potatoes 48, and honey bees 32. This number is known as the diploid number of chromosomes. The sex cells (sperm and egg) have half the number of chromosomes and are said to be haploid. Honey bee drones, because they develop from unfertilized eggs, have a haploid number of chromosomes which is 16. In the formation of sex cells, chromosome pairs stretch out together. While lying close to each other, the chromosomes can exchange portions of their DNA, which may contain one or more genes. This process is known as crossing over and is responsible for most of the variation seen in progeny.
When we try to make a bee stock more useful, we are trying to select the DNA within that population - increasing the frequency of genes responsible for desirable traits. In doing so, however, we affect other gene frequencies, and this can create problems. So the trick is to be able to modify genetic material without losing diversity.
The following are what I see to be main barriers to bee stock improvement.
- Controlling Mating Behaviour
When the virgin queen is about six days old, and the weather conditions are fine, with light winds and temperatures of at least 20 degrees C, she flies out a considerable distance from her hive to mate. Research has shown the average distance between apiary and mating place is two kilometers (1.6 km = one mile). She has been shown to fly out as far as five km. Mating flights last between five and 30 minutes. She will make between one and three flights. The number of flights she makes depends on the concentration of spermatozoa in her spermatheca. Once it reaches a certain concentration, she will not fly again. To achieve this concentration she must mate with several drones - at least seven, maybe many more.
The virgin does not fly haphazardly about hoping by sheer chance to encounter drones. She goes directly to drone congregation areas. Exactly why drone congregation areas originate where they do is not fully understood, but the same areas are used year after year. The drones within any congregation area come from many different colonies, and probably several different apiaries. Drones have a flight range of up to six kilometers with flights of five common. Thus drones can range over an area of roughly 78 square kilometers.
It has been shown that virgins very rarely mate with related drones, which reduces the chances of inbreeding, one of the perils to avoid in any controlled breeding scheme. Thus, if we allow virgins to mate naturally, we have no control over the drones. Even with isolated mating yards, control is not absolute. What other plant or animal breeder has to make an attempt at genetic improvement with only control over 1/2 of the genetic equation? To compound this there are multiple matings. Each virgin mates with seven or more drones, and thus the colony is made up of seven or more sub-families.
- Retaining Sex Alleles
In most sexually reproducing organisms, sex determination is governed by a sex chromosome. In honey bees, however, sex is determined by a single gene. This gene has many variants or alleles, maybe as many as 18. One should feel lucky, however, to maintain 10 or so in a breeding population. It works like this, if two different alleles come together at fertilization, a female (worker or queen) results. Drones are haploid and have one allele. However, if two of the same allele come together, a diploid male results. We never see diploid drones in the hive because when only a few hours old, they are cannibalized by the workers. Evidence of this is a hole (spot) in a slab of newly capped worker brood.
As the number of sex alleles decreases, the more likely it is that two of the same allele will come together, increasing the number of diploid drones. As the percentage of diploid drones produced increases, so does the spottiness of the brood. There is an obvious impact on a colony's productivity, therefore, when some well-intentioned bee breeder reduces the number of sex alleles in a queen. Even if such queens are of high physiological quality and genetic potential, they are handicapped because a percentage of their eggs are not viable.
The mechanics of heredity and the mating behavior of the honey bee, therefore, are geared to genetic diversity. The mechanism of sex determination in honey bees also penalizes any narrowing of this natural diversity.
- Reducingn Inbreeding Depression
Hybrid vigor results when two unrelated members of a species are crossed. The vitality of the progeny usually exceeds that of either parent. This is also known as heterosis, a mostly unexplained increase in life force. The crossing of unrelated parents results in many more genes carrying two different alleles. When a pair of genes consists of different alleles the resulting organism is said to be heterozygous. A generalized increase in heterozygosity is responsible for triggering heterosis. The opposite state is when genes carry two of the same allele. These organisms are said to be homozygous.
A reduction in heterosis occurs with inbreeding. An "inbreeding depression" is triggered as the percentage of homozygous genes increases. This results in an unexpected loss of vigor--sluggish colony build-up, loss of disease resistance, decreased production, and higher winter loss.
Inbreeding depression can result from selections over generations for the best genetic combinations. The breeder's downfall is increasing the percentage of homozygous genes in too small a population. This is not always apparent to a producer who is selecting breeders from perhaps hundreds of colonies. Unfortunately, it is not the size of the test population, but the number of breeding queens used, which determines how quickly inbreeding depression develops.
- Maximising Quantitative Traits
The characteristics we are trying to improve in honey bees are quantitative traits. These may involve many genes, each contributing only small effect. Compounding this is the fact that these traits are not those of a single breeding individual (the queen) but, instead characterized in a colony composed of many sub-families.
It is fortunate that many important economic traits such as honey production and winter hardiness in bee populations, even though they are hugely complex, and controlled by a large number of genes, do show good response to selection. However, once these selections cease, any increase in traits which has been achieved is lost very quickly as gene frequencies return to pre-selection balances. Thus, maximizing quantitative traits is a continuous process which must be done with great care.
- Minimising Environmental Variation
Evaluations must reliably identify the genetically superior individuals in the test population in order to increase quantitative traits. However, because colony performance is evaluated in the field, it is difficult to control environmental influence. Possibilities to reduce environmental effects consist of equalizing colonies before evaluations begin, minimizing drift; and eliminating evaluations between apiaries. Finally, because a queen's physiological quality itself can have a major effect on some aspects of colony performance, queens undergoing evaluation must be uniform in age and condition.
- Minimising the Influence of Racial Hybrids
Even if we implement all the suggestions above, and put into evaluations the care and effort required, it is all for naught if the genetic superiority we identified with our evaluations is not heritable. Unfortunately, the increased vigor provided by heterosis cannot be inherited.
We have two races of honey bee in New Zealand, the Dark European honey bee and the Italian. Even though most of the bee breeding effort goes into maintaining commercial bee stocks as Italian, the reality is that most of the colonies are to varying degrees racial hybrids. Racial hybrids can be great, and through hybrid vigor, are often productive. However, they are of no breeding value, and provide only false leads to someone carrying out colony evaluations.
To get anywhere, we have to breed true to race -- whatever that race is. The Dark European honey bee drones appear to be very aggressive in the drone congregation areas because they appear to have a mating advantage of almost Africanized-bee-like proportions. So the only way to keep a test population true to race is to have absolute control over the mating using Instrumental Insemination.
- Keeping an Open Mind
It may be a lot cheaper to import a silk purse, than to try and make one out of a sow's ear. Taking advantage of different races and breeding work done overseas by importing genetic material could save time and money and be a dramatic shortcut to better bees. Times have changed, importations of genetic material can be done safely, whether they be semen or breeder queens.
- Keeping On the Road to Better Bees
It is possible to breed better bees. Results of 23 years of selections in Germany with Carniolans demonstrates this. The progress was slow, but it was progressive, and it was done without the recent research into closed population bee breeding. Results of this technology are more impressive. As an example, the Western Australian Department of Ag. bee breeding program which ran from the mid-80's up until this year realized a 10% increase in honey production per year. There is a similar program up and running in N.Z. It is totally self-funded by its participants, and has a legal structure of a limited liability company. There are 25 participants and each is an equal shareholder in the company. For their investment, they receive improved breeding stock.
The New Zealand program's goal is to improve an Italian-type bee in a closed population, maintaining 25 lines - one for each participant. Matings are absolutely controlled using instrumental insemination. The semen is collected in equal volumes from each line, then pooled together and homogenized.
The program's structure has all the necessary ingredients leading to better bees. The foundation stock was contributed from all over the country to maximize genetic diversity, and once the population is closed off, maintaining 25 lines with mother daughter replacements each generation means that the program can run for more than 25 generations without the introduction of any stock from outside the program and still maintain sufficient diversity to avoid any ill-effects from either homozygous sex alleles or inbreeding depression.
The use of pooled and homogenized semen has been a most important breakthrough in bee breeding. It maximizes genetic diversity and selection pressure. The semen dose each queen receives represents all 25 lines being maintained in the program. Because the semen, although amazingly diverse, is homogeneous, each queen receives an equal genetic dose. Therefore, any genetic variation uncovered by the evaluations, is maternal in origin. This gives the selections more meaning, increasing the potential rate of improvement.
Instrumental insemination makes maintaining racial purity a piece of cake, you just don't collect semen from darkish drones. Selections for winter hardiness and honey production are carried out in two parts. The first part is done by the participants, and is more broadly based. Each participant receives the daughters of one line in mid--February each year. They are introduced into equalized units and equalized again in April. The queens are evaluated for their overwintering ability using the criteria of colony weight loss between May and September (New Zealand's winter). They are also scored on brood viability, temperament, hygienic behavior, and color.
In late September, the participants choose the two best queens, based on the above evaluations, to be introduced into hives in one large dispersed apiary. They are equalized and in December (New Zealand's summer) and evaluated for honey production potential using short-term colony weight gain. An initial and a final weight are taken 10 days apart. The queen which performed best for each line is chosen and becomes the breeder for the next generation. 10 daughters are reared from each breeder and these are inseminated with pooled semen. It is these daughters which form the test population for next season's evaluations.