Cannabis Chronicles

Our favorite introduction to plant breeding was penned by Carol Deppe. It’s a common sense guide to seed saving, and we find it quite inspirational. It’s full name is - Breed Your Own Vegetable Varieties: The Gardener’s & Farmer’s Guide to Plant Breeding & Seed Saving - and we couldn’t give it a higher recommendation for aspiring plant breeders.

Here’s one of the subjects covered, an excerpt on Maternal inheritance, which seems quite appropriate for a Mother’s Day entry.

Maternal Inheritence

The pericarp, the outermost layer of the seed, is maternal, not embryonic tissue. So it’s phenotype reflects the genotype of the mother, not of the embryo the seed contains. Some other genes whose products operate at the very early stags may also show maternal inheritance – in other words, the phenotype for the grown plants is established by the genotype of the mother. Maternal inheritance of this sort is Mendellan, but the expected ratios appear one generation later.

The Cytoplasmic in Inheritence

More than 99% of the genes in plants are arranged on chromosomes located in the the nuclei of cells, and they are inherited according to Mendel’s laws. However, some small fraction of genes are located in the organelles in the cytoplasm, and they aren’t inherited in this way. Even though there aren’t very many extrachrosomal or cytoplasmic genes, they have a disproportionate importance in plant breeding.

The inheritance of genes in cytoplasmic organelles is completely non-Mendellian. It is almost always exclusively maternal. The egg is a big cell with a lot of cytoplasm. The pollen is a very small cell with little cytoplasm, which is not normally transferred to the egg during fertilization. Nuclear DNA comes equally from both parents, but cytoplasm comes only from the maternal parent.

Once in a while you can do a cross where one of the characteristics you are interested in is associated with a gene that is cytoplasmic instead of nuclear. Suppose you do a cross between a variety that has mottled coloration pattern on the leaves and one that does not. If you cross a mottled female to a normal male, some or all of the F1 plans are likely to be mottled. But it you cross a mottled male with a normal female, none of the F1 plants will be mottled, nor will any offspring in any future generations.

The cytoplasm is relevant to us in additional ways. The cytoplasm of a variety and it’s nuclear genome have evolved to be compatible. When distant relatives are crossed, sometimes the cytoplasm and the nuclear genome in the F1 or of some of it’s F2 progeny aren’t compatible. In extreme cases lethality results. In less extreme cases the plant may be sterile or pollen-sterile. Male sterile cytoplasms have, in fact, been useful in the production of hybrids. Such cytoplasms aren’t absolutely male-sterile; they are sterile only in the presence of one or more nuclear genes.

Anytime we are crossing two fairly distant relatives, we need to realize that the reciprocal crosses might be genuinely different. Both will give rise to the same nuclear genotype, but they will set it down in the middle of different cytoplasms. This could have a dramatic effect on the phenotype of the F1 and subsequent generations.

Many modern crops trace their nuclear genomes to only a dozen or so sources. But it is common for vast fractions of a modern crop to have only one cytoplasmic genome. The southern corn blight in the United States in the early 1970’s was caused by the blight sensitivity of a particular cytoplasm – a male sterile cytoplasm that was in virtually the entire commercial crop, whatever the nuclear genome involved.

Whenever we do a cross using a particular variety as the maternal parent, we are preserving it’s cytoplasm and discarding that of the male. I think we should pay more attention to preserving and increasing the diversity of the cytoplasms in our food crops. All we have to do to preserve a particular cytoplasm is to make sure that we use it as the female parent when we do crosses.

Carol Deppe is a plant breeder and writer who holds a BS degree in Zoology from the University of Florida and a PhD in biology from Harvard University