Apomixis is the replacement of sexual fertilization with asexual reproduction. With normal sexual fertilization, embryos are formed from the union of male and female gametes, which produces genetically varied offspring. In contrast, apomictic embryos are formed without paternal contribution.
Apomixis is the means of reproduction found in Hosta ventricosa, the only Hosta species which produces offspring apomictically. The asexually-produced offspring are generally believed to be identical to the parent plant.
Mark Zilis studied the reproduction process of H. ventricosa in a tissue culture laboratory and provided the following observation:
"After the egg is fertilized and a zygote is formed, surrounding tissues in the endosperm begin forming additional embryos. Unlike the zygote (the result of an egg being fertilized by pollen), these embryos are asexually formed and are exact copies of the mother plant. In tissue culture studies I undertook in the early 1980s, I found anywhere from three to ten asexual embryos in addition to the sexual embryo. The latter is relatively large, while the asexually formed embryos are of various sizes. Occasionally, the sexual embryo dies, leaving only the asexual embryos to develop. What are the implications of this process? It means that germination of a single Hosta ventricosa seed results in multiple plants. One is the result of fertilization while the others come from apoximis. In soil, usually only a few of the apomictic seedlings develop, but in vitro, i.e., in a tissue culture environment, it is possible to bring all of the asexual embryos to maturity." The Hostapedia, page 12
Further in his book, Mark wrote that "...Hosta ventricosa produces most of its seedlings through apomixis. The resulting asexually produced seedlings are identical to the mother plant. For such apomixis to occur, however, fertilization must take place, allowing for an occasional sexually produced seedling with slight differences. A few of these have been introduced to the trade, including ‘Holly's Honey’, ‘Little Blue’, and ‘Lakeside Black Satin’." The Hostapedia, page 989
While the majority of seedlings produced by Hosta ventricosa are the result of apomictic reproduction, it should be remembered that some of the offspring are from normal sexual reproduction and therefore are not identical to the original plant. In a conversation with Bill Meyer, he told me of significant variations among a group of H. ventricosa seedlings he had observed years ago. Bill wrote,
"Being apomictic does not mean being 100% apomictic - only that a percentage of the seed is clonal. The rest are sexually formed, so that gives you a mix in the seedlings of the apomictic plant. A lot of H. ventricosa seedlings have been raised to date, in part because of the ‘comes true from seed’ story that goes around. Many of them clearly look different, but I’m not sure how much phenotypic variation you get on apomictic seed. Do they end up identical like TC divisions or are there significant differences? Years ago, I walked rows of H. ventricosa seedlings at a large wholesale nursery and saw quite a bit of difference. For example, quite a few had funnel-shaped flowers rather than the expected bell shape."
But what portion, if any, of those seedlings were apomictic? Could it be that some of the seedlings Bill observed were the result of sexually formed embryos and that not all were formed by apomixis? Even if it could be determined that a particular group of H. ventricosa seedlings were the result of apomixis, there may still be slight differences, as there is some variation within a species.
There is much more that we have to learn about Hosta ventricosa and its reproductive process. Is the same method of asexual reproduction passed on to its seedlings? Do the sports of H. ventricosa such as ‘Aureomaculata’ and ‘Aureomarginata’ produce offspring through apomixis as H. ventricosa itself? Is the variegation displayed by H. ventricosa sports passed to apomictically formed offspring? Is the ability to reproduce apomictically ever passed on when H. ventricosa is used as a pollen parent?
We have many questions and few answers, so more research will have to be undertaken in the future. Currently, we know that H. ventricosa reproduces through apomixis, that it is the only Hosta species to do so, and that offspring are formed both through sexual fertilization and through asexual fertilization as well.
Your input and comments are appreciated, please use our contact page.