The process of biological sex determination in humans may be familiar to most of us.

People with ovaries generally have two X chromosomes. In contrast, people with testes typically have two different sex chromosomes, one X and one Y. However, sex determination in humans, just like we will see in bees, is not as simple as males are XY and females are XX. In humans, we see these variations with intersex individuals who have genitalia that does not match their chromosomes, have testicular and ovarian tissue, or have a combination of X or Y chromosomes other than XX or XY.

Figure 1.Sex determination by sl-CSD in haplodiploid bees, different colored chromosomes carry different alleles at the sex determining locus (Created with BioRender.com).

In other organisms, sex determination is even more variable. Let us consider the diverse class Reptilia. Snakes and lizards determine sex based on their sex chromosomes at fertilization, but for many turtles and all Crocodilians, environmental conditions determine sex. You may be familiar with temperature-based sex determination in sea turtles and conservationists' work to ensure that populations maintain a sustainable sex ratio under the increased temperatures resulting from climate change. Much like the class Reptilia, insects have a diverse suite of sex determination pathways. In this module, we will explore the mechanism of sex determination in bees, the importance of sex ratio, and how these processes impact bee conservation.

Sex Determination

All hymenopterans, including ants, wasps, and bees, use a system called haplodiploidy, where male bees are (typically) haploid and female bees are diploid. Female bees can choose the sex of their offspring since male offspring arise from unfertilized eggs and female bees from fertilized eggs.

The term “haplodiploidy” is a bit of a misnomer for the best described mechanism of sex determination in Hymenoptera, single-locus complementary sex deter- mination (sl-CSD). Although the specifics are nuanced, the difference between the simplified description of haplodiploidy (males are haploid and females are diploid) and sl-CSD may be important for under- standing how population genetics impact bee conservation.

In the sl-CSD system, sex is determined by the zygosity of a single sex determining locus. In most large, outbred populations there are many alleles at the sex determining locus – sometimes as many as 20. Thus, if an individual is diploid (having two sets of genes, one inherited from their mother and the other inherited from their father), that individual will most likely have two different alleles, meaning they are heterozygous. These diploid, heterozygous individuals are females. In Hymenoptera, most males are haploid and thus hemizygous at the sex determining locus. However, when popula- tions become inbred such that there is low genetic variability at the sex determining locus, you can find individuals that are diploid but homozygous (Figure 1). One might think that these individuals would be female but in fact they end up developing into males who are reproductively sterile (see Sex Ratios, Haplodiploidy, and Bee Conservation below).

Figure 2. The proportion of offspring which are daughters is significantly lower when floral resource levels are low (p = 0.020) (Figure adapted from Kim 1999).

Mason Bee Sex Ratio and Sex Allocation

Haplodiploid sex determination provides bees with a unique ability to choose the sex of their offspring in response to external (environmental) conditions and as well as intrinsic conditions, such as their age and overall health. We briefly discuss several of these topics in the context of mason bee behavior in the Foraging Behavior and Nesting and Mating Behavior modules. One of the most notable and easily observed manifestations of this is the non-random distribution of offspring within the nests of stem-nesting bees, such as mason bees. In nearly all stem- and cavity-nesting bees, females preferentially place their female off- spring at the back of the nest and their male offspring at the front of the nest. Because males (typically) emerge before females, this arrangement allows the males to exit the nest before their sisters.

The females of many bee species, including mason bees, can also adjust the sex ratio of their brood in response to environmental conditions. Female bees will tend to lay more male eggs when conditions are bad and more female eggs when conditions are good. Floral resource scarcity, inadequate nesting materials, poor weather, and the presence of brood parasites can push female bees to lay more unfertilized, haploid (male) eggs (Figure 2). Female offspring require more food (floral resources) to reach a healthy adult size than male offspring, which are small compared to adult females. When floral resources are deficient within the foraging area, females tend to create smaller provisions and lay more haploid male eggs. (The alternative, diploid female eggs laid on undersized pollen provisions, can reduce female body size and increase female mortality.)

Intrinsic conditions, such as age and overall health, may also alter how females allocate resources. As actively nesting female bees near the end of their lives, they tend to exhibit reduced foraging efficiency and vigor. In response, female bees create smaller pollen provisions with insufficient nutrition for healthy female bee development. As a result, across the female bee’s life, there is a shift from laying female eggs towards laying more male eggs. Additionally, unmated females will exclusively lay haploid male eggs.

Figure 3. A diploid male vortex is a positive feedback loop which can drive a population towards extinction (Figure adapted from Zayed and Packer 2005).

Sex Ratios, Haplodiploidy, and Bee Conservation

The unique genetics of sex determination in bee populations create interesting conservation genetics dynamics. Perhaps most simply, is the manipulation of sex ratios within the population. In general, a greater ratio of males to females has little impact on population growth, and conversely, this makes female mortality significantly more impactful on population dynamics. An ideal ratio in mason bees is between 1.5 to 2 males per female. Having approximately two males per female helps limit the number of unmated females, which could result in a more male biased population, limit population growth, and reduce pollination, which is almost exclusively performed by females.

Several other hypotheses have been proposed regarding the impact of haplodiploidy on bee conservation, some suggest a benefit to bee populations others do not. One possible benefit is the purging of deleterious, or harmful, mutations from the population by haploid males. In recessive traits, diploid individuals may not experience the effects of a deleterious mutation by expressing the functional gene on the homologous chromo- some, haploid individuals are unable to compensate for the harmful mutation since they only have one copy of the gene. However, this also may reduce their ability to respond to environmental or biological pressures since they only have one allele for each gene, whereas the allelic variation within diploid females may make them more resistant to similar pressures.

Overwintering Adult Nest Dissection— Hands on

The location of male and female bees in mason bee nest and the ratio between the sexes can teach us alot about sex allocation in mason bees. In this activity, we will examine the pattern of male and female adult mason bees within a nest.

BACKGROUND

Sex Determination Module PDF

FIGURES

ACTIVITIES

Sex Determination Activity PDF 

Sex Determiantion Activity Answers PDF

Figure 1. Osmia cocoons are separated in the nesting reeds by mud walls.

Figure Front: Female Osmia lignaria

Figure Side: Female Osmia lignaria

Figure Front: Male Osmia lignaria

Figure Side: Male Osmia lignaria 

RESOURCES