Haplodiploid Sex Determination

Summary

This script delves into the fascinating world of sex determination across various species, highlighting the XY system in humans and the diverse mechanisms in others like spiders, butterflies, birds, and honeybees. It explores the discovery of honeybee genetics by Johannes Json, the haplodiploidy system, and the role of the CSD gene. The narrative also touches on the importance of population genetics, the maintenance of sex allele diversity, and the evolutionary implications of polyandry in honeybee queens.

Takeaways

• 🧬 Human sex determination is based on the combination of X and Y chromosomes: XX for females and XY for males.
• 🕷 Spiders have an XO system where males lack a Y chromosome, and XX indicates females.
• 🦋 In butterflies, males are ZZ and females can be ZW or have missing sex chromosomes, unlike the human XY system.
• 🐦 Birds have a ZW system where males are homogametic and females are heterogametic.
• 🐢 Turtles exhibit temperature-dependent sex determination, where the environment dictates the sex of the offspring.
• 🐝 Honeybees have a haplodiploid sex determination system where males develop from unfertilized eggs and are haploid, while females are diploid.
• 🏆 Johannes Json proposed that male honeybees develop without fathers, a discovery that led to the understanding of honeybee sex determination.
• 🧬 Honeybee sex is determined by a single gene, the CSD gene, which has multiple alleles that can combine to form either viable females or diploid males.
• 🐝 Diploid male honeybees are not viable and are consumed by nurse bees shortly after hatching.
• 🧬 The honeybee population maintains a balance of different sex alleles through natural selection, favoring rare alleles to prevent frequent production of diploid males.
• 🐝 Queens mate with multiple males to increase brood viability and reduce the chances of producing diploid males, supporting the evolution of polyandry.

Q & A

How do humans determine sex?

In humans, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). A person inherits an X chromosome from both parents; if both are X chromosomes, the individual is female, and if one is a Y chromosome, the individual is male.

What is the term 'gametes' in the context of sex determination?

Gametes are the reproductive cells (sperm and eggs) that participate in sexual reproduction. Females produce larger, stationary gametes (eggs), while males produce smaller, motile gametes (sperm).

How does the sex determination system differ in spiders compared to humans?

In spiders, the sex determination system is XO for males (missing a Y chromosome) and XX for females. This is different from humans, who have XY for males and XX for females.

What are the different sex chromosome combinations in butterflies?

In butterflies, males can be ZZ (two of the same chromosomes), while females can be either ZW (two different sex chromosomes) or ZO (missing one chromosome).

How does the environment influence sex determination in clownfish?

In clownfish, sex can change based on environmental conditions and sexual competition. They can transition from female to male or male to female depending on these factors.

What external factor determines the sex of turtles?

In turtles, sex is determined by the temperature of the soil in which the eggs are incubated, which is an external signal that dictates whether they develop into males or females.

What did Johannes Json discover about honeybee sex determination?

Johannes Json discovered that honeybee males have no fathers. He conducted experiments with unmated queen bees that still produced males, suggesting that males develop from unfertilized eggs.

What is the term 'haploidy' in relation to honeybee sex determination?

Haploidy refers to having a single set of chromosomes. In honeybees, males (drones) develop from unfertilized eggs and thus have only one set of chromosomes, making them haploid.

What did PW Whiting propose about sex determination in parasitic wasps?

PW Whiting proposed that a single gene is responsible for sex determination in parasitic wasps. He observed diploid males, which led him to conclude that there must be a gene that determines sex, with different alleles leading to either male or female development.

What did Otto Mackensen discover about honeybee sex determination?

Otto Mackensen proposed that honeybees have a similar sex determination system to parasitic wasps, with a gene series that determines sex. He suggested that matings between a queen and a drone with matching sex alleles would produce lethal diploid males.

What did Jaroslaw Cebra discover about diploid honeybee males?

Jaroslaw Cebra discovered that diploid honeybee males are not lethal but are consumed by nurse bees shortly after hatching. He demonstrated this by grafting drone larvae into queen cells and then transferring them back to drone combs, where they developed into diploid males.

What is the significance of the CSD gene in honeybee sex determination?

The CSD gene is the single gene responsible for determining the sex and gender of honeybees. It has multiple alleles, and different combinations of these alleles result in either diploid females or diploid males, with the latter typically being consumed by the colony.

Outlines

00:00

🧬 Sex Determination Systems in Nature

This paragraph discusses the various mechanisms of sex determination in different species. In humans, females have XX chromosomes, while males have XY. The concept of gender is introduced based on the type of gametes produced, with females producing larger, non-motile gametes and males producing smaller, motile ones. The paragraph highlights the diversity in sex determination systems across species, such as spiders (XO for males, XX for females), butterflies (ZZ for males, ZW or ZO for females), birds (ZZ for males, ZW for females), and turtles where sex is determined by environmental temperature. The paragraph also touches on the unique case of honeybees, where males develop from unfertilized eggs and are haploid, while females are diploid. The historical discovery by Johannes Json that honeybee males have no fathers is mentioned, marking a significant milestone in the study of sex determination.

05:03

🐝 Honeybee Sex Determination and Diploid Males

The second paragraph delves into the specific case of honeybees, where the sex determination system was proposed by PW Whiting to involve a single gene responsible for sex. Whiting observed diploid males in bone wasps, which contradicted the haplodiploid model where males are haploid. Otto Mackensen's work on honeybees suggested that queens mated with drones carrying similar sex alleles would produce diploid males, which he initially thought were lethal. However, further research by Jared C. Bąk showed that diploid males were not lethal but were consumed by nurse bees after hatching. This paragraph also discusses the identification of the complementary sex determining gene (CSD) by Martin Beye, which is responsible for the sex and gender of honeybees, and the existence of multiple alleles of this gene that can lead to different outcomes in honeybee reproduction.

10:05

🔬 Population Genetics and Honeybee Sex Alleles

This paragraph explores the population genetics aspect of honeybee sex determination, focusing on how a multitude of sex alleles are maintained in a population. It explains that rare alleles are favored by selection because they less frequently produce diploid males when combined with other alleles. The paragraph discusses how the frequency of sex alleles can change over generations due to the selective pressures against common alleles that lead to more frequent production of diploid males, which are then consumed by the colony. It also describes an experiment by Bąk, where diploid male larvae were reared in queen cells to avoid being consumed, and how this practice helped to understand the dynamics of sex allele frequencies in honeybee populations.

15:07

🐝 Polyandry and Brood Viability in Honeybees

The final paragraph discusses the implications of the sex determination system on the evolution of polyandry in honeybees. It explains how the number of matings a queen undergoes affects the distribution of brood viability in colonies. With fewer matings, there's a higher chance of mating with a male carrying a similar sex allele, leading to a higher proportion of diploid males and reduced brood viability. As the number of matings increases, the probability of matching matings decreases, thus increasing overall brood viability. This insight is crucial for understanding why honeybee queens mate with multiple males, as it maximizes the genetic diversity and viability of the offspring, which is beneficial for the colony's survival and health.

Keywords

💡Sex determination

Sex determination refers to the biological process that leads to the development of an individual's physical and physiological characteristics associated with their sex. In the context of the video, it is discussed as a mechanism that varies across species, with humans having an XY system and honeybees having a unique haplodiploid system. The video script uses sex determination to explain the genetic basis of gender in different organisms.

💡Haploid and Diploid

Haploid refers to cells that have one set of chromosomes, while diploid cells have two sets. In the video, these terms are crucial for understanding honeybee sex determination, where males (drones) are haploid, developing from unfertilized eggs, and females (queens and workers) are diploid, resulting from fertilized eggs.

💡XO and XY

These terms describe different sex determination systems. In the video, the human XY system is contrasted with the spider's XO system, where the absence of a Y chromosome (XO) determines maleness. This illustrates the diversity of sex-determining mechanisms in the animal kingdom.

💡Homogametic and Heterogametic

Homogametic individuals produce gametes (eggs) of the same type, while heterogametic individuals produce gametes of different types. In the video, it is mentioned that in honeybees, females are heterogametic (producing both X and Y gametes), and males are homogametic (producing only one type of gamete), which is a key aspect of their haplodiploid sex determination system.

💡ZW and ZZ

These terms represent another type of sex-determining system, where ZW indicates female (with two different sex chromosomes) and ZZ indicates male (with two identical sex chromosomes). The video uses this system to highlight the variability of sex chromosomes across species, such as in butterflies.

💡Haplodiploidy

Haplodiploidy is a sex-determination system where males develop from unfertilized eggs (and are thus haploid) and females develop from fertilized eggs (and are diploid). The video explains that honeybees use this system, with males being haploid and females being diploid, which is central to understanding their unique reproductive biology.

💡Environmental sex determination

This concept refers to the influence of environmental factors, such as temperature, on the sex of an organism. The video mentions that in turtles, the temperature of the soil where eggs are incubated determines the sex of the hatchlings, demonstrating how external conditions can play a role in sex determination.

💡Polyandry

Polyandry is a mating system where a female mates with multiple males. The video discusses the evolution of polyandry in honeybees, suggesting that it may be linked to the maintenance of genetic diversity and the avoidance of producing diploid males, which are not viable.

💡Sex alleles (A1, A2, etc.)

Sex alleles are different forms of a gene that determine sex. In the video, the script explains how different combinations of sex alleles in honeybees can result in either viable females or non-viable diploid males, highlighting the complexity of the honeybee's sex-determining system.

💡Complementary sex determining gene (CSD)

The CSD gene is a single gene identified in honeybees that is responsible for determining sex and gender. The video script describes how this gene, along with the various sex alleles, contributes to the honeybee's unique haplodiploid sex determination system.

💡Diploid males

Diploid males are males that have two sets of chromosomes, which is unusual in species with an XY or ZW sex-determination system. The video explains that in honeybees, diploid males are produced when a queen mates with a male carrying a sex allele identical to one of hers, but these males are not viable and are consumed by the colony.

Highlights

Humans have an XY sex-determination system, with females having XX and males having XY chromosomes.

Sex determination systems vary widely among different species, with genetic and environmental factors playing roles.

Spiders use an XO system, with males being XO and females being XX.

Butterflies exhibit variability in sex determination, with males being ZZ and females being ZW or ZO.

Birds have a ZW system, with males being ZZ and females being ZW.

Ctenophore, or comb jellies, can change sex based on environmental conditions and sexual competition.

Turtles' sex is determined by the temperature of the soil where their eggs develop, demonstrating environmental sex determination.

Johannes Json proposed that male honeybees have no fathers, based on experiments with unmated queen bees producing males.

Honeybees have a haplodiploid sex-determination system, with females being diploid and males being haploid.

Haplodiploidy is found in about 20% of all animal species, including honeybees.

PW Whiting proposed a single gene responsible for sex determination in bony wasps, which was a significant discovery in the field of genetics.

Otto Mackensen developed instrumental insemination technology and proposed a sex determination mechanism in honeybees involving lethal genes.

Jared C boa discovered that diploid male honeybees are not lethal but are consumed by nurse bees after hatching.

Martin Beye identified the complementary sex determining gene (CSD) in honeybees, which is responsible for sex and gender.

The CSD gene has multiple alleles, with 19 different forms found in one population study, leading to a complex understanding of sex determination.

Selection favors rare alleles in honeybees, maintaining genetic diversity and preventing the dominance of common alleles.

The number of matings by a queen bee affects the distribution of brood viability in honeybee colonies, influencing the evolution of polyandry.

Transcripts

we're all familiar with the mechanism of

sex determination in humans uh females

have two X chromosomes uh males have an

X and A Y uh if you inherit an X

chromosome from your mother and an X

chromosome from your father you're a

female however if you ex inherit an X

chromosome from your mother and a y

chromosome from your father you're a

male different sex determination systems

uh determine gender

in uh in different species uh gender is

a term that we used to identify

individuals on the basis of the gam

meets that they produce individuals that

produce the larger stationary or IMM

modal gametes we assign the the gender

of female individuals who produce the

small motile gametes we call males what

they have for underlying

genetic sex determination uh can be

highly variable for example if you look

at

spiders uh males are uh XO not XY

they're missing a y chromosome so XO is

a male XX the homogametic sex uh they

are

females in butterflies it's a little

more variable the males get two of the

same chromosomes we call those Z

chromosomes they're so they're

ZZ and the females can either be ZW they

have two different sex chromosomes or

they can be

Z not no chromosomes so they can be

absent one of the chromosomes as the

case of the the uh male spiders but this

is in this case it's female

butterflies uh the birds they have a uh

a system whereby

the males are the homo gametic sex they

get two identical chromosomes and the

females are the heterogametic sex they

get two different

chromosomes clownish can change sex they

can go from being a female to a male or

a male to a female depending upon the

environment that they're in and the

sexual competition that they're that

they're

encountering Turtles uh sex is

determined in turtles by the temperature

of the soil in which the eggs develop so

it's a very completely external signal

that determines whether they develop

into a male or a

female in

1945 Johannes Json proposed that male

honeybees have no fathers he determined

this by doing a series of

experiments um where he had uh colonies

where the Queens had not

mated uh and he found that they still

produced males so he figured that there

was no fathers because the Queens hadn't

M it therefore um they had to be have

the males derived uh uh without being

the eggs not being

inseminated uh the pictures I have there

are it's a place in Germany that we

stayed and I was going to tell you a

side story but I I'll make it real brief

the the person who owned the place where

we're staying there uh his wife

great great great uncle was Johan jarson

and one afternoon she showed us all

those medals there that you see on his

chest um she had them in a box and uh

showed them to us but Jaron can be

considered the the father of honeybee

genetics and not only when he discovered

the sex determining system of honeybees

being uh one where uh females have a

father and males don't uh it was the

first proposed sex determination system

for any

animal honeybees have a hlo diploid

mechanism of of of sex

determination

um Hao diploidy is not rare 20 about 20%

of all animal species have this

particular kind of sex determining

system uh as you can look look at the

figure you can see that

females uh are derived from uh the Egg

of the mother and the sperm of the

father and they have two sets of

chromosomes they get one set from the

mother one set from the father whereas

males uh only have one set of

chromosomes that set that they inherited

from their mother so you see the male on

the lower left that's the phenotype that

comes about after it develops and on the

right you get the females which can

either be queens or workers depending

upon the the U nutritional feeding

program that they get while they're

developing in 1933 PW Whiting proposed

uh a a a mechanism whereby one gene is

responsible for determination of sex in

boned wasp which is what he studied he

did a studies and he he found that some

of the individuals that were that were

produced were diploid males mean he knew

was H that they were haplodiploid so he

expected there that males would all be

haid like with honeybees one set of

chromosomes and females would be diploid

but when he started looking closely he

found that in fact uh some of the males

that were produced had two sets of

chromosomes so he figured out the only

way this could happen is if there's a in

the way that they segregated it had to

be one gene responsible for determining

sex and he determined that in order to

be a female you had you were heter zygus

for this Gene you had two different

alals of this Gene and if you had the

two of the same you were a diploid male

in this case the diploid male survived

that they couldn't reproduce because

their their their sperm weren't viable

uh for uh uh passing on

Offspring

1951 um Otto

mackinson uh he was also the one of the

developers of the instrumental

insemination technology shown on the

right he proposed that in fact honeybees

have a similar

system and he proposed that um

that a queen that was made to a drone

that had a sex that matched one of her

two would produce 50% diploid

males uh so half of their brood would be

homozygous and half would be

heterozygous and the diploid males he

said were lethal he thought they just

died so he called it Lethal lethal genes

and he determined this when he did these

crosses because here on the left you see

the The Brood pattern of a queen that um

uh received a sex Al that was diff

different from either of the ones that

she had uh and on the right you see The

Brood pattern of one who received a uh

was inseminated by a male that had a sex

Al that was identical to one of her two

so in this case here you got

50% loss of brood over here you don't

have the loss of brood so the what the

one on the right was called shot brw and

that was one way of determining whether

you had um a queen who had mated

with uh too many males that had sex

alals similar to

hers so then he went on showed that if

you if you look at this series um that

there's a whole series of Al he called

it a lethal series of

alals on the left you have this queen

she has X1 X2 those are her two sexal

they're different she's

heterozygous and she made it with a male

that's an X X1 male so with the X1 male

if you look down at the bottom here its

homozygous produces diploid males she

also made it with uh X3 X4 X5 say and if

you look at the um the offspring that

are derived from them uh they don't

produce deployed males so he he looked

at this and he determined there's got to

be a whole set of these different Al uh

in some sort of an alic series but again

he called them lethal it was a lethal

Series in

1963 Jared C boa who a Polish uh

apicultural

scientist uh did a really clever

experiment he started looking at larvey

drone

larvey of different different

ages and he discovered that when they're

first hatched from the egg he could he

could look at the Drone larvey and he

could make a distinction with those that

were were apparently

diploid then if you look later uh they

were missing so right after hatching you

could you could find larvey that were

different and were distinguished

that were diploid males and then they

disappeared so you never saw any diploid

males so then he was curious are they

lethal did they die or was just

something else happen so he took those

larv the the the newly hatched larve of

drones uh in drone cells and he grafted

those larve into cells like this that

are used for raising

Queens when he put them in those Queen

cells and gave them back to the

bees the bees actually continue to feed

them they didn't kill them and then

after about 3 days he took those lar out

of the cells they're being raised in the

queen cells and he put them back into

cells uh in in in drone comb give it

back to the bees and they raised them

and when they came out they were diploid

males he he did genetic studies and

showed that they were in fact diploid so

he came to the conclusion it's not a

lethal series what's happening is is the

viable diploid males are being consumed

by the the nurse bees shortly after they

hatch the nurse bees can detect

differences between the hloy males and

the diploid males and they eat the

diploid males so you never see them

unless you go to this trouble of uh of

producing

them in 2003 Martin baa the guy on the

right uh published a paper in uh in cell

I actually was an author on it too uh

where we had identified

the complimentary sex determining Gene

named it CSD the single Gene that was

responsible for um

determining uh the sex and gender of uh

of honeybees and in subsequent studies

Martin showed that in a population allog

together this alic series consists of

and one the one population he studied 19

different Al so there were 19 different

for forms of this one Gene and in

combination with each other he found

that um there were 171 active

combinations of these these 19 different

sexal in other words if you put if you

put A1 with

A3 uh it was a viable female if you put

A1 with A4 was a viable but maybe A1

with a18 even though they were they were

different Al when you sequenced them

they were actually fun functioning as if

they were the same gene um but anyway he

went through all these combinations and

he found out that all in all there were

19 inactive combinations and

171 active combinations that gave you

diploid females and not diploid

males so for this section this is

population genetics and I it's important

in understanding kind of the effects of

of the sex determining system on

populations uh I have here this is

thinking cap you need to put this on uh

this is a little bit more dense a little

more difficult to understand the

concepts but we'll get through it anyway

we'll

try so how do you maintain so many

different sexules in a

population and each of these sexules in

a population tend to go towards equal

frequency so if you have 10 sex they

tend to all be on10th frequent or

something something close to that so

they they tend to reach an equilibrium

state of being equally frequent if you

have five they're equally frequent each

one of them is is uh present in the

population at one5 um so how does that

come about well it comes about because

selection favors rare alals So an Al

that's rare will in

combination with the other alals less

often produce a diploid male because

there just aren't that many copies of it

out there so overall in the beginning

when there's a lot of them around

they're really favored and so the

frequency of that a will increase in the

population from one generation to the

next ones that are too frequent too

often when when you match a queen mates

if she's got a really common Al she will

too often mate with another male that

has an Al like hers that common Al and

then those individuals will will not be

viable females they will be deployed

males that are consumed by the workers

and eliminated from the

population on the upper left we show

that's a that's a drone congregating

area uh there's a comet of drones in

there and this just shows the different

mixture of of sexal I say most of these

drones have the A1 sexil okay the queen

flies through this drone congregating

area and she mates with drones mat with

different males and then she comes back

to the Colony and she starts laying eggs

um so assuming the B the female was A1

A2 she has two different sexal she has

to and say she made it with uh five

males three a1s and an A2 and an A3 then

the progeny resulting from the

combination of her two sexal that go

into eggs one in one one sexal into each

egg but they they get distributed across

the eggs in combination with the sperm

from the five different males gives you

these

genotypes uh derived from it as you can

see the more of the A1

genotypes are

eliminated uh than the others

U so there's going to be selection in

this case that's going to be selection

against the a1s and in favor of the A2

a3s so in the next generation there's

going to be fewer a1s and more a2s and

a3s so what's the probability of a

matched mating when a queen flies

through a drone conar what's the

probability she's going to mate with a

male that has one like hers let's assume

there's 10 sexes in the population and

they're equally frequent so you have X1

through X10 each one of those is present

at a frequency of

10% the queen has two different sex

let's just say X3

X6 each time she mates there's a 20%

chance she will mate with a matching a

Leal she has two different ones and each

of them in the population are at 10% so

that's each time she mates there's a 20%

chance that the male she mates with will

have an Le identical to one or the other

of

hers matings with a matching

maale produces 50% deployed drums

so every time she M mates with one at a

you know chance 20% chance for each

mating and there's a 20% chance that

there will be 50% brood viability

derived from that particular male that

she mated

with so if she mates one time there's a

20%

chance that she will lose 50% of her

viable brood due to homozygosity at the

sex Locus there's an 80% chance that

she'll mate with one that has a

different Al from hers and she'll have

100% this figure demonstrates what

happens as you increase matings the

upper leftand corner that's a queen

mates there's 10 sexal they're equally

frequent Queens mate one

time in this

population Queens at mate if you look at

Queens at mate one time 20% of them will

have 50% brood viability 80% of them

will have

100 if you go to mate two times now

there's three different classes if you

mate if you mate with

um two males that have matching matching

Al to

yours uh the probability of that is is

relatively

low uh if there's three matings uh you

would have 50% brute VI if you if you

made it with one male that hasn't

matching Al and one male that doesn't

you're going to have 75% that's a little

higher probability and if you mate with

two males neither one of which uh have

an Al that matches one of yours then

it's a probability of 60 something

percent so you can see how the number of

matings affects the distribution of

brood viability in different colonies

across the population and then when you

get up to 10 10 meetings there's a very

low probability that you're going to get

um uh enough matching matings you're

going to have very low low brood

viability and uh there's a probability

reasonable probability that you're going

to have uh matings with males we're

going to give you a reasonably High

viability of

brood this is important for

understanding the evolution of polyandry

the evolution of um one of the

hypothesis for the evolution of

polyandry multiple mating of Queens