Chapter 9

Reproductive Strategies

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A honeybee hive. In most populations of honeybees, a queen can reproduce by laying either haploid eggs that produce sons or fertilized diploid eggs that produce daughters.
Photo by Konrad Wothe/age fotostock.

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CHAPTER CONCEPTS

  • Reproduction can be sexual or asexual.
  • Organisms can evolve as separate sexes or as hermaphrodites.
  • Sex ratios of offspring are typically balanced, but they can be modified by natural selection.
  • Mating systems describe the pattern of mating between males and females.
  • Sexual selection favors traits that facilitate reproduction.

The Sex Life of Honeybees

Honeybees (Apis mellifera) have a complicated sex life. They live in hives that may contain tens of thousands of bees, usually progeny of the same mother, known as the queen. Like many organisms, the queen bee produces sons and daughters, but she does so in a rather unique way. Early in her life, the queen bee flies out of the hive and mates in the air with a group of male bees. Male bees, known as drones, are smaller than the queen. A queen mates with several drones, but the larger drones contribute more sperm than the smaller drones. The queen stores sperm from this single mating event in a special organ in her body, known as the spermatheca, where it remains viable for several years. She uses these sperm to fertilize her eggs and make diploid daughters, known as workers. In contrast, she creates drones by laying unfertilized haploid eggs. The drones a queen produces rarely mate with her, but instead mate with other queens outside the hive. After mating, the drones die.

A key issue for a successful hive is the proper sex ratio of drones and workers. In a typical beehive, the queen bee may produce a few dozen drones but tens of thousands of workers. Since the workers do the vast majority of the work in the hive, producing many more workers than drones is in the best interest of the queen. Since workers only live for 4 to 7 weeks, the queen must constantly produce more of them.

“The Cape honeybee can lay diploid eggs without ever mating.”

Genetically, the workers and the queen are quite similar. They are both female and they both arise from a fertilized, diploid egg. What makes them different is the food they are given as larvae. For the first few days of life, all larvae are fed royal jelly, a liquid produced by the worker bees, but after this the larvae destined to be workers are switched to a diet of honey and pollen. These workers are not capable of mating with a drone, but they can lay unfertilized eggs. Larvae destined to be queens continue to be fed royal jelly, which allows the future queen to grow very large. The queen’s size allows her to eventually lay as many as 2,000 eggs per day.

When a beehive experiences the decline of its queen, new larval queens are often already being produced as replacements. Sometimes, however, the death or departure of a queen happens unexpectedly and there are no replacements. In such cases, some of the workers lay eggs but, since the workers cannot mate, their eggs are haploid and destined to turn into drones. Without a queen to lay fertilized eggs, the colony will eventually die.

Scientists have recently discovered an exception to this scenario in a subspecies of the honeybee known as the Cape honeybee (Apis mellifera capensis) that is found in southern Africa. Cape honeybee workers can lay diploid eggs without ever mating and, therefore, can ensure the persistence of the beehive. Researchers recently discovered that a single recessive gene controls this ability. As a result, depending on which alleles they carry, individual workers can lay either haploid eggs that give rise to drones or diploid eggs that give rise to workers.

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The complexity of honeybee reproduction serves as an excellent example of the variety of reproductive strategies that have evolved. These options include reproducing with or without a sexual partner, choosing the number of mating partners, selecting the best traits of a sexual partner, varying the way the sex of offspring is determined, and controlling the number of sons versus daughters. In this chapter, we will explore the wide range of reproductive strategies in a diversity of organisms.

SOURCE: H. M. G. Lattorff, R. F. A. Moritz, and S. Fuchs, A single locus determines thelytokous parthenogenesis of laying honeybee workers (Apis mellifera capensis), Heredity 94 (2005): 533–537.

The evolution of reproductive strategies involves a large number of different factors, many of which are influenced by ecological conditions. For example, organisms can evolve to reproduce sexually or asexually; each strategy has unique costs and benefits, particularly, as we shall see, when species interact with parasites and pathogens. Organisms can evolve to reproduce as separate sexes or as hermaphrodites that possess both female and male sexual organs. If they reproduce as hermaphrodites, they must also evolve solutions to the problems associated with self-fertilization. In many species the sex ratio of offspring can be altered to respond to changing ecological conditions. Finally, we see many different mating strategies to improve fitness, including the number of mates and the preference for certain traits in the opposite sex. This chapter explores how ecological conditions affect the evolution of sex and the strategies that organisms have evolved to increase their fitness.