This post is an to answer Eric Weber‘s recent question about male sterility in seeds. Eric had been thinking about using Green Goliath broccoli in a breeding project but heard that High Mowing Seeds has been having a hard time producing Green Goliath seed and suspected male sterility. He wondered what caused this problem and what to do about it.
The disclaimer: This post is kind of heavy on genetics, that may or may not affect its reading enjoyment for some. Also, I am not on expert on this topic and might be inaccurate or downright wrong on some of my understanding – I don’t think I am that far off though.
Well, what is male sterility?
From Wikipedia: “Male sterility is the failure of plants to produce functional anthers, pollen, or male gametes.”
A male sterile plant cannot pollinate other plants or itself. However, the female reproduction structures of the flower are still fertile. A male sterile plant can therefore set seed if they are pollinated by a different (male fertile) plant. This genetic anomaly is used in hybrid seed production since it guarantees cross-pollination on male sterile plants.
There are 3 male sterile situations:
- Genetic Male Sterility
- Cytoplasmic Male Sterility
- Genetic Cytoplasmic Male Sterility.
1. Genetic Male Sterility
In this case, male sterility is caused by a recessive gene (ms). When both parents carry the recessive gene then the pollen is not viable. If you cross a genetical male sterile plant (msms) with a male fertile plant (MsMs), the F1 offspring will only carry one copy of the ms gene (Msms) and will be fertile. However, when you cross two of these F1 plants you will find the following genotyoes: MsMs, Msms, and msms. There will be some male sterile plants (msms) in addition to plants carrying the ms gene without expressing it (Msms).
I don’t know how easy it is to identify the male sterile plants in a large highly cross-pollinating population of a crop like broccoli. But if you can, and you rogue male sterile plants out, over many generations you will greatly reduce the frequency of the ms gene. This might be adequate to build a strong population.
If Green Goliath is genetical male sterile, Eric can probably use it in his breeding project as a mother plant. From what I’ve read though, genetic male sterility is not very common.
2. Cytoplasmic Male Sterility (CMS)
The cytoplasm is the stuff inside a cell wall in which the cell nucleus floats around in. Here’s a drawing. When a plant sets seed, some DNA from the mother plants cytoplasm is carried along. Cytoplasmic Male Sterility is transmitted by this type of DNA. If the mother was CMS, the offspring will be also.
This type of male sterility will shoot you in the foot if you are trying to breed an open pollinated population since there will be no gene flow from the CMS plants to any of the other plants.
3. Genetic Cytoplasmic Male Sterility
This is a CMS plant for which exist genes that can be introduced from another plant which will restore male sterility. This is great if you have plant breeding facilities and genetic material to work with but most of us farm scale breeders probably don’t have these resources.
If Green Goliath is either cytoplasmic male sterile or genetic cytoplasmic male sterile, it might be best for Erik to find some other broccoli variety to work with.
What to do?
Well, if you know (or suspect) your variety is male sterility, then plant a row of the male sterile variety beside a row of a male fertile variety. The male sterile should set seed but all of the seed will be crossed with the male fertile.
Save the seed from each row separately,
The following year, only sow seed saved from the male sterile row.
If the plants don’t set seed, they are CMS. Your breeding project likely shuts down here.
If seed does set, the plant are probably genetic male sterile. The hard work begins now. The ms genes are still present in the population just waiting to be expressed. If you can identify male sterile plant in future generations, you can rogue them out than over time. Some head to row selection in 5-6 generations might help clean up this genetic nightmare.
Many hybrid seed is produced using male sterility. Especially onions, carrots, beets, and some brassicas. If you try to dehybridise such a variety, you might wind up with fertility problems sooner or later.
Now wasn’t that fun.
Eric, Good Luck