Haplodiploid breeding system

Willing to die for their sisters: worker honey bees killed defending their hive against yellowjackets, along with a dead yellowjacket. Such altruistic behaviour may be favoured by the haplodiploid sex determination system of bees.

According to inclusive fitness theory, organisms can gain fitness not just through increasing their own reproductive output, but also that of close relatives.

In evolutionary terms, individuals should help relatives when Cost < Relatedness * Benefit. The requirements for eusociality are more easily fulfilled by haplodiploid species such as bees because of their unusual relatedness structure.

In haplodiploid species, females develop from fertilized eggs and males from unfertilized eggs. Because a male is haploid (has only one copy of each gene), his daughters (which are diploid, with two copies of each gene) share 100% of his genes and 50% of their mother’s. Therefore, they share 75% of their genes with each other.

This mechanism of sex determination gives rise to what W. D. Hamilton termed “supersisters”, more closely related to their sisters than they would be to their own offspring.

Workers often do not reproduce, but they can pass on more of their genes by helping to raise their sisters (as queens) than they would by having their own offspring (each of which would only have 50% of their genes), assuming they would produce similar numbers.

This unusual situation has been proposed as an explanation of the multiple (at least 9) evolutions of eusociality within Hymenoptera.

Haplodiploidy is neither necessary nor sufficient for eusociality. Some eusocial species such as termites are not haplodiploid.

Conversely, all bees are haplodiploid but not all are eusocial, and among eusocial species many queens mate with multiple males, creating half-sisters that share only 25% of each-other’s genes.

But, monogamy (queens mating singly) is the ancestral state for all eusocial species so far investigated, so it is likely that haplodiploidy contributed to the evolution of eusociality in bees.

Eusociality

A Western honey bee swarm

 

Western honey bee nest in the trunk of a spruce

Bees may be solitary or may live in various types of communities. Eusociality appears to have originated from at least three independent origins in halictid bees.

The most advanced of these are species with eusocial colonies; these are characterised by cooperative brood care and a division of labour into reproductive and non-reproductive adults, plus overlapping generations.

This division of labour creates specialized groups within eusocial societies which are called castes. In some species, groups of cohabiting females may be sisters, and if there is a division of labour within the group, they are considered semisocial.

The group is called eusocial if, in addition, the group consists of a mother (the queen) and her daughters (workers).

When the castes are purely behavioural alternatives, with no morphological differentiation other than size, the system is considered primitively eusocial, as in many paper wasps; when the castes are morphologically discrete, the system is considered highly eusocial.

True honey bees (genus Apis, of which seven species are currently recognized) are highly eusocial, and are among the best known insects.

Their colonies are established by swarms, consisting of a queen and several hundred workers. There are 29 subspecies of one of these species, Apis mellifera, native to Europe, the Middle East, and Africa.

Africanized bees are a hybrid strain of A. mellifera that escaped from experiments involving crossing European and African subspecies; they are extremely defensive.

Stingless bees are also highly eusocial. They practise mass provisioning, with complex nest architecture and perennial colonies also established via swarming.

A bumblebee carrying pollen in its pollen baskets (corbiculae)

Many bumblebees are eusocial, similar to the eusocial Vespidae such as hornets in that the queen initiates a nest on her own rather than by swarming.

Bumblebee colonies typically have from 50 to 200 bees at peak population, which occurs in mid to late summer. Nest architecture is simple, limited by the size of the pre-existing nest cavity, and colonies rarely last more than a year.

In 2011, the International Union for Conservation of Nature set up the Bumblebee Specialist Group to review the threat status of all bumblebee species worldwide using the IUCN Red List criteria.

There are many more species of primitively eusocial than highly eusocial bees, but they have been studied less often.

Most are in the family Halictidae, or “sweat bees”. Colonies are typically small, with a dozen or fewer workers, on average. Queens and workers differ only in size, if at all.

Most species have a single season colony cycle, even in the tropics, and only mated females hibernate. A few species have long active seasons and attain colony sizes in the hundreds, such as Halictus hesperus.

Some species are eusocial in parts of their range and solitary in others, or have a mix of eusocial and solitary nests in the same population.

The orchid bees (Apidae) include some primitively eusocial species with similar biology. Some allodapine bees (Apidae) form primitively eusocial colonies, with progressive provisioning: a larva’s food is supplied gradually as it develops, as is the case in honey bees and some bumblebees.

Solitary and communal bees

A leafcutting bee, Megachile rotundata, cutting circles from acacia leaves

Most other bees, including familiar insects such as carpenter bees, leafcutter bees and mason bees are solitary in the sense that every female is fertile, and typically inhabits a nest she constructs herself.

There is no division of labor so these nests lack queens and worker bees for these species. Solitary bees typically produce neither honey nor beeswax.

Bees collect pollen to feed their young, and have the necessary adaptations to do this. However, certain wasp species such as pollen wasps have similar behaviours, and a few species of bee scavenge from carcases to feed their offspring.

Solitary bees are important pollinators; they gather pollen to provision their nests with food for their brood. Often it is mixed with nectar to form a paste-like consistency. Some solitary bees have advanced types of pollen-carrying structures on their bodies.

Very few species of solitary bee are being cultured for commercial pollination. Most of these species belong to a distinct set of genera which are commonly known by their nesting behavior or preferences, namely: carpenter bees, sweat bees, mason bees, plasterer bees, squash bees, dwarf carpenter bees, leafcutter bees, alkali bees and digger bees.

A solitary bee, Anthidium florentinum (family Megachilidae), visiting Lantana

Most solitary bees nest in the ground in a variety of soil textures and conditions while others create nests in hollow reeds or twigs, holes in wood.

The female typically creates a compartment (a “cell”) with an egg and some provisions for the resulting larva, then seals it off. A nest may consist of numerous cells.

When the nest is in wood, usually the last (those closer to the entrance) contain eggs that will become males. The adult does not provide care for the brood once the egg is laid, and usually dies after making one or more nests.

The males typically emerge first and are ready for mating when the females emerge. Solitary bees are either stingless or very unlikely to sting (only in self-defense, if ever).

The mason bee Osmia cornifrons nests in a hole in dead wood. Bee “hotels” are often sold for this purpose.

While solitary, females each make individual nests. Some species, such as the European mason bee Hoplitis anthocopoides, and the Dawson’s Burrowing bee, Amegilla dawsoni,are gregarious, preferring to make nests near others of the same species, and giving the appearance of being social.

Large groups of solitary bee nests are called aggregations, to distinguish them from colonies. In some species, multiple females share a common nest, but each makes and provisions her own cells independently. This type of group is called “communal” and is not uncommon.

The primary advantage appears to be that a nest entrance is easier to defend from predators and parasites when multiple females use that same entrance regularly.

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