Written by Joe Ballenger
We’ve gotten a lot of questions about boy and girl bugs. Nancy wrote a post titled Is that Bug a Boy or a Girl, which covered some general rules about how boy and girl bugs are different. We also got a question about how this works in bees.
Awhile back, we got a question about how Butterflies make boys and girls…and these guys do things a little bit differently than the other critters we’ve discussed.
Sex determination in insects is really cool, because there are so many different ways to make boys and girls. Each type of insect has a different twist on the same thing, and different groups within these types can even make boys and girls in completely different ways.
This is also really important for a lot of reasons. Some mutations which result in pesticide resistance are sex-linked in some insects, and this influences how these genes move through the populations. From a technological perspective, you need to know how boys and girls are made so you can make sterile insects. Sterile bugs would be great because they show great promise in reducing pesticide applications in agriculture, especially in developing nations.
Even though this question was asked about Monarchs specifically, this is really important stuff to know!
So how do butterflies and moths make boys and girls?
So the first thing to get out of the way is this…what are the sex ratios in monarch butterflies?
Many insects have skewed sex ratios for a number of reasons. Maybe the males are more likely to get eaten, maybe there are more males produced than females…stuff like that. Monarch adult populations tend to have more males than females, but the sex ratios are about even at birth.
If you’re interested in the reasons the adults have skewed sex ratios, Frey & DeLong (1993), and Nylin et. al 1994 go into more details about why this happens.
XY, ZW, ZO…and Z-Counting
The short answer to the question in the post above is this: sex determination in Monarchs happens at the time of fertilization.
However, to understand why this is the case you have to know a little bit about how butterflies and moths make boys and girls. The system they use is actually pretty familiar…but with a twist.
In our bee post, we explained that humans have a XX/XY sex-determination system. Males, like Joe, have X and Y sex chromosomes. Females, like Nancy, have two copies of X chromosomes. Developmentally, the human fetus starts out as a female until male-specific genes on the Y chromosome get turned on relatively late in development.
Butterflies and moths are a little bit different, but there are a lot of similarities. Like humans, they have sex chromosomes. Like humans, the sex of the offspring is set at fertilization.
Well, kind of on that last point. We’ll discuss that more in a minute.
ZW: Most butterflies and moths…including Monarchs
Butterflies and moths have a sex determination system that’s almost the exact opposite of humans. Males have two Z chromosomes, and females have the ZW combination. Without the W chromosome, butterflies turn out male.
In humans, sperm cells differ from one another by carrying different combinations of chromosomes. Sperm can carry either an X or a Y chromosome, which means the sex of the offspring is determined by the male.
In butterflies, it’s the egg which decides the sex of the offspring. All the male sperm cells carry the Z chromosome, while the eggs can carry either a Z or a W chromosome. In butterflies, the sex of the offspring is determined by the female.
Most butterflies and moths have the ZW system. That’s the way it works in most species. Most.
One family stands out as radically different from all the other butterflies and moths…and that’s the family Micropterigidae. Micropterigids are weird, because they’re a sort of transitional species. They have a lot in common with the Caddisflies, which are the closest relatives to the butterflies and moths. Their common name-the archaic moths-reflects this.
Caddisflies and Micropterigids share a common sex determination system where males have two copies of sex chromosomes, and females only have one copy of sex chromosomes. On paper, it looks a little bit like this:
Female eggs may or may not have a sex chromosome. If an egg containing the Z chromosome gets fertilized, it becomes male. If an egg without the sex chromosome gets fertilized, it becomes female.
A lot of butterflies and moths have a system which relies on multiple Z chromosomes. Or multiple W chromosomes. That’s a really complicated situation…too complicated to get into here. Simplistically, it’s the number of Z chromosomes which seems to matter in some groups. This is called ‘Z-counting’.
Okay, but, when does sex determination happen?
The easy and simple answer is that sex determination happens during fertilization, but there are times where butterflies and moths will produce different ratios of boys and girls.
Temperature matters in captivity
If you keep your monarchs in a refrigerator, these low temperatures can cause sex chromosomes to be divided unevenly when gametes are made. This usually causes a female bias in reared specimens. On the converse, very high temperatures can cause male-biased sex ratios in reared specimens. However, this isn’t how people typically keep these butterflies. It’s more of an interesting aside.
Another way the offspring’s sex can be manipulated is by infection with a parasitic bacterium called Wolbachia. Wolbachia is a bacteria which can’t get into sperm cells very well, so it makes butterflies produce more female offspring than males.
Wolbachia is a very weird bacteria, because it can kill male offspring before they hatch. It’s also capable of turning male individuals into fully functioning biological females. How it does this in butterflies isn’t well understood, because the gene that it messes in other insects with isn’t found in butterflies. Figuring out which genes the insects use to make males and females is really hard in butterflies, because the tools which work in other insects don’t work in butterflies.
Wolbachia can have pretty drastic effects on butterfly populations. There are some butterfly species which have been almost driven to extinction by Wolbachia, so this is very important to conservation programs.
The Bottom Line
Monarch butterflies don’t have a biased sex ratio, except as adults. However, if you’re pulling small numbers of eggs off a plant you might get artificially skewed numbers because of chance.
The sex of butterflies and moths is determined at the time of fertilization. What sex the offspring becomes depends on whether the sperm cell fertilizes an egg carrying the Z chromosome, or an egg carrying the W chromosome. This is what happens in Monarchs, and most other butterflies and moths.
There are some things, namely Wolbachia infection, which can cause butterflies to produce offspring that are entirely one sex, and that would happen after fertilization in this case. There isn’t a whole lot of data on Wolbachia infection in Monarchs, but there is some evidence that some individuals harbor this bacteria. This evidence comes from an unpublished PhD thesis, so further study is needed to confirm these results. It’s not known how widespread this is, or what effects they have on the populations.
Frey, D. F., & Leong, K. L. (1993). Can microhabitat selection or differences in’catchability’explain male-biased sex ratios in overwintering populations of monarch butterflies?. Animal behaviour, 45(5), 1025-1027.
Nylin, S., Wickman, P. O., & Wiklund, C. (1995). An adaptive explanation for male-biased sex ratios in overwintering monarch butterflies. Animal behaviour, 49(2), 511-514.
Swamy, R. S. (2012). The butterfly Danaus plexippus is infected with the bacteria Wolbachia and Spiroplasma (Doctoral dissertation, Emory University).
F. (2007). Sex chromosomes and sex determination in Lepidoptera. Sexual Development, 1(6), 332-346.