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Written by | Kestrel
Honey bees are well-known social insects that are often used to describe hard laborers. A typical colony consists of a queen bee (also known as a queen bee), a small number of male bees, and a large number of worker bees. Social bees have a strict division of duties, with queen bees and males directly involved in sexual reproduction and responsible for passing on generations, while worker bees are responsible for childbearing, nesting, foraging and defense. ("The Song of The Strike Worker" is played here)
In order to maintain the balance of the bee colony in composition, the reproduction of bees is strictly regulated. Worker bees and queen bees hatch from fertilized eggs and are both females, while males hatch from unspotilized eggs – that is, males have no fathers.
In general, the offspring resulting from sexual reproduction inherit a set of genetic material contributed by both the father and the mother. Whether it is a worker bee, a queen bee, or a human, they all have two sets of chromosomes, which are "diploids" (diploid). The difference is that worker bees and queens have 16 pairs of chromosomes (32 in total), while humans have 23 pairs of chromosomes (46). The male bee hatched from an unfertilized egg has only half of its normal chromosomes (16 in total), or haploid (haploid, who left it without a father).
Although worker bees and queens have identical chromosomes, only queen bees have reproductive function. Scientists have found that the reproductive function of worker bees is suppressed, mainly due to food. Originally, the queen bee has always eaten royal jelly as food, while the worker bee only ate royal jelly for three days after birth, and can only rely on pollen and nectar to feed later. Royal jelly has substances that regulate gene expression, allowing queens who have been eating royal jelly to develop a mature reproductive system. In addition, the queen also secretes exosterone (pheromone), which further inhibits the reproductive ability of worker bees. So worker bees are actually female bees with incomplete reproductive organs.
However, under conditions of prolonged absence of the queen bee, the ovaries of the worker bees can be activated. But they still do not have the ability to mate, and can only lay unfertilized eggs, which develop into male bees. This phenomenon of parthenogenesis is also called parthenogenesis. The offspring of single-sex reproduction of worker bees are all males, hence the name arrhenotokous parthenogenesis (arrhenotoky). These males can mate with the queens of other colonies and pass on the genome they carry.
Before the colony is divided, the worker bees will build 4-10 "queen cells" in the hive, and the larvae in them will be fed with royal jelly and complete their development almost at the same time. Generally speaking, a population retains only one queen bee, and the strongest queen of these queens will become the new owner, and the other queens will be killed. After the new queen bees are born, it is time for reproductive swarming. The old queen will leave with some of the worker bees and establish a new colony.
Correspondingly, because as soon as they come out, they face a life-and-death contest, and the sooner they climb out of the throne, the more favorable it is for the candidate queen. The post-bee that needs to mature as soon as possible is the fastest (15-16 days), the worker bee is the second (21 days), and the male is the slowest (24 days).
Unlike humans, bees have male and female, but no sex chromosomes, and their sex determination mechanism is more special, called haplodiploidy, which is mainly found in hymenoptera insects and some other arthropods. But because of the great diversity of insects, this sex-determining mechanism accounts for 20% of all animal species, so it is not so special.
So, what exactly determines the sex of bees?
At the molecular level, there is a site in the honeybee genome called csd, which is the main genetic locus that controls the sex of bees, and the full name is complimentary sex determiner. In bee populations, there are about 19 different versions of the genes at this loci, or 19 alleles. Male wasps are haploid and naturally have only one csd gene. Queen bees and worker bees are diploids, there are two csd genes, and the versions of the two csd genes are always different (called heterozygotes), because bees with the same version (called homozygotes) die early in development.
Thus, the CSD site is the most upstream signal that determines the sex of the bees: csd heterozygosity, which determines whether it is female; csd homozygote, the bees cannot survive; only a single csd gene determines that it is male.
There are also special cases, for example, in highly inbred experimental populations, diploids with csd sites in homozygous develop into male bees, but such males are either infertile on their own or sterile triploid offspring produced by mating.
Such a mechanism of gender determination is compatible with the "eusociality" of bees. The so-called true sociality means that the population should have three characteristics at the same time: reproductive division of labor, generational overlap (mature individuals in the group can be divided into more than two generations, such as grandfathers and fathers are alive at the same time) and cooperative childbearing.
The altruistic behavior of worker bees can be explained by the theory of kinship selection. The relatedness coefficient (r) is a measure of affinity between individuals, defined as the probability that two individuals share a gene and both copies come from the same ancestor (e.g., the common gene of mother and child comes from the mother's father, the child's grandfather; the sister's common gene comes from their mother). In the case of worker bees, the correlation coefficient r=0.5 between an egg-laying worker bee and a haploid male bee it lays, because the male bee inherits half of the mother's genetic material and has a 1/2 probability of having a gene of its mother.
The kinship coefficient between the worker bee and the male bee born after the bee is only r=0.25, because both are the offspring of the queen bee: the probability that the male bee has a gene after the queen is 1/2, and the probability that the worker bee, as the sister of the male bee, also has this gene is also 1/2 (it may also come from another male bee), so the probability that the gene shared by both is 1/4 from the queen bee is 1/4.
However, there will not be only one worker bee in the colony to lay eggs, and the kinship coefficient of one worker bee and all the male bees born of worker bees is r≈ 0.125, so that the male bees that take care of other worker bees are instead of taking care of the male bees that are born after the bees - why take care of the male bees with lower relative coefficients? Of course, the higher the r, the more "own person". Therefore, in the ordinary bee colony, the worker bees themselves do not lay eggs, and even remove the eggs laid by other worker bees.
Sounds pretty damaging isn't it? In fact, this behavior of the worker bee is to increase its own general fit. The so-called fit is the ability to produce surviving offspring, and the generalized fit is even lower: you don't have to produce offspring yourself, and other people's offspring contain the same genes as yourself. (My heart fluttered: What a cute little creature!) ）
More than a decade ago, however, beekeepers in South Africa found themselves "subverted" by some worker bees that could lay their own eggs. What's even more dramatic is that these strange worker bees produce the same offspring as their own genomes, which is equivalent to cloning themselves! These bees are called cape honeybee, a subspecies of the Western honey bee (scientific name Apis mellifera). Its Latin literary name is Apis mellifera capensis, which distinguishes it from the subspecies Apis mellifera scutellata adopted by the general beekeeping industry.
Figure 1 "Blackened, but not completely blackened". A larger proportion of black on the abdomen is a convenient feature for distinguishing cape bees.
The genomes of these two subspecies are almost identical. However, the typical worker bee mentioned at the beginning of this article is only likely to lay a haploid male bee, while a cape bee can lay diploid eggs on its own and develop into a worker bee, or, if it occupies the royal stage, it will develop into a queen bee - you heard it right, the queen born by the worker bee! This phenomenon is called parthenogenesis (thelytoky).
Seeing this, you may recall the scene in Resident Evil Extinction: in front of Alice and the cloned Alice, more cloned Alice is suspended in a nutrient solution, multiplied in an endless pile of lattice egg containers (Figure 2). You may recall that in Resident Evil Final Chapter, the aging Alicia leaves her memories to her clone, the protagonist Alice, who gives her a complete "personality" and hopes that she will survive instead of herself. In fact, Benjamin Oldroyd, an entomologist at the University of Sydney in Australia, also uses "reincarnation" (literally, "reinventing the flesh") to refer to the process by which the worker bee, who cannot mate, gives birth to his own queen clone.
Figure 2 Scenes from the movie Resident Evil Extinction.
Indeed, over the past 30 years, millions of cape bee clones have frustrated the South African beekeeping industry. When mixed with populations of other subspecies, cape bees are so-called "social parasites." These worker bees not only "illegally" lay eggs and compete for the throne, but also emit external hormones for ordinary worker bees to feed them as young bees, almost enjoying the treatment of queen bees. They monopolize reproduction, causing the original population to gradually collapse and then invade new colonies.
Cape colonies can also survive the absence of queens for months until a new queen is produced. In the 1990s, 50,000 colonies a year in South Africa were destroyed, and cape bees were embargoed in Europe, America, Australia and other places. The social parasitic phenomenon of cape bees was first discovered after the introduction of South African honey bees from Brazil, but fortunately it was quickly processed at that time, otherwise it might not be called cape bees.
Figure 3. The geographical distribution of cape bees is very narrow, limited to the southernmost part of the African continent (the green part).
As we all know, the body of the bee is black and yellow striped, but in the abdomen of the cape bee, the proportion of black is greater (Figure 1), which allows us to easily identify it. Cape bees who can appoint their own leaders (queen bees) and produce their own armies can be regarded as blackened workers.
How do cape bees pass on the full set of chromosomes to their offspring?
The eggs of cape bees also undergo a second division of the meiosis, but after the chromosome isolation is completed, the nuclear membrane and cell membrane are not formed immediately. Four pairs of chromosomes, i.e. four pronuclei, are lined up together, and then the two pronuclei in the middle (containing two sets of chromosomes from non-sister chromatids) fuse ("central fusion") and revert to diploid. The worker bees of cape bees always perform parthenogenesis, while queens always undergo sexual reproduction.
Figure 4. Comparison of the breeding patterns of the general bees with the cape bees.
Parthenogenesis has drawbacks, the most prominent of which is the loss of genomic heterozygosity, which reduces the adaptability of species. In the middle of the first division of the meiosis, the chromosomes recombine and then go through the aforementioned "central fusion" process. If a chromosome fragment of a gene has been exchanged with a fragment of the corresponding non-sister chromatid, the fragment will be the same copy on both sets of chromosomes in the egg that will eventually develop into a female. For either gene loci, 1/3 of the heterozygosity should be lost, because during the "central fusion" process, any one of the chromatids is selected, and one of the remaining three will be exactly the same as it.
Loss of heterozygosity is not conducive to survival. An important reason is that some defective recessive genes will show adverse traits. For example, heterozygous deletions are often found in cancer, and tumor suppressor genes are often found in the missing areas. Researchers have thought that cape bee colonies should decline quickly. But more than thirty years later, this has not happened, perhaps because cape bees have social parasitic properties. The researchers speculate that in addition to the homozygous lethal effect of the csd gene, there should be other unknown mechanisms that maintain the genomic heterozygosity of cape bees. In short, the cape bee carries many mysteries and is still waiting for us to solve.
Final question, when did such a rebellious "strike worker" as the Cape Bee appear?
The researchers found that all of these cape bees are descendants of the same mutant worker bee, which probably lived in 1990. You may ask, how exactly is this found? Here we need to mention an important research method of ecology - satellite DNA.
Satellite DNA is a highly repetitive class of genetic sequences. It is a molecular marker that is commonly used in ecology to represent longer chromosome fragments. It is called "satellite" because in genomic DNA cesium chloride density gradient centrifugation experiments, it is always distributed outside the main strip, as if the position of the satellite.
There is a class of satellite DNA called "microsatellites", which consists of a sequence of 2-4 bases that is repeated in large numbers. Analyzing a large number of microsatellite sites from Cape bee samples, the scientists found that all but a few mutations were present at these microsatellite sites, either heterozygous of the only two versions of the microsatellite sequence, or homozygous of one of the two versions. This means that they all come from a mutant bee, all on the parthenogenesis lineage of this "little ancestor"—the real-world Smith.
It sounds a bit scary, but you can also understand the matter from a different perspective. After all, the cape bee mutation is a small probability event, and the probability of such an event occurring once is of course much greater than the probability of occurring independently twice (the latter probability is the second power of the probability of the former).
In fact, the geographical distribution of cape bees is very narrow, limited to the southernmost corner of the African continent (Figure 3). For beekeepers in this area, the cape bee is a lovely presence – easy to take care of, more adaptable to the local ecology, more tolerant of tile mites (Varroa destructor, the leading pest of the beekeeping industry), and a complete alternative to ordinary bees in commercial value. For researchers, the cape bee is one of the most interesting bee subspecies.
bibliography
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