CactiGuide.com

So I have one domino plant that flowered (while I was out of town of course!) with two flowers. I believe it would have flowered at night, since my boyfriend noticed the flowers in the morning, and they wilted away later in the day.
Now the flower stems are wilted but there's two big lumps at the bottom, which I can only assume to be fruit. How is it that this cactus produced fruit when there was only one plant? I assume the flowers would have been pollinated by insects, but is Echinopsis subdenudata self-fertile? I saw the chart in one of the other posts in this forum (a downloadable .doc file), and it marked the E. subdenudata as self-sterile.
So what's up with the fruit? Do many self-sterile cacti produce fruit that do not have seeds or that have seeds which are sterile? Or does a cactus only produce fruit if it has been successfully pollinated, and the fruit contains fertile seeds?

These plants are self-sterile, but several things could have happened:

1. Since these plants often time their flowering by the moon cycle, there could have been another plant flowering within a mile or so that donated pollen via an energetic moth. In this case, you will have fertile seeds once the fruit swell and split open.

2. Pollen from a related plant could have made it to these flowers and created seeds that will grow into hybrids. This may not be as likely, but it is also not impossible because this is the most commonly kept species of decorative Echinopsis in a lot of places.

3. Sometimes plants "think" they have been fertilized and begin to form fruit, but that fruit ultimately has no viable seeds. I've had this happen on other Echinopsis (Lobivia) but I am not actually sure how common it is.

4. Your individual plant could have some kind of mutation that allows for self-fertilization. This is extremely uncommon, but it does happen. I have one Gymnocalycium that always fertilizes itself and they're not "supposed" to.

Keep a watch on the fruit, and keep us updated in a week or two about how they've done.

Here's another two possibilities:

5. Perhaps your plant is of hybrid origin (i.e. not entirely E. subdenudata) and it's got some genes from a self fertile species (supposedly there are some self fertile Echinopsis)

6. Some self sterile plants become self fertile in the presence of another plant's pollen. Harrisia jusbertii is a good example of this. I'm not so sure Echinopsis is like this since it seems to like to form hybrids though

I think 2 or 3 is the most likely, and mine are more remote possibilities. Were any of your other cacti in flower?

My E. ancistrophora, the cactus in my avatar picture, is closely related to E. subdenudata, and also supposed to be self-sterile - but it regularly produced seed. Fertile seed, too - I've got seedlings growing.

Though they're adapted for moth pollination, that might not be the only thing that pollinates them. Mine was ignored by bees, but regularly visited by Syrphid Flies. They're small enough that they crawled right down the floral tube to get to the nectar. Of course, they got covered with pollen.