The (one ) explanation, that come to mind. could be that if you planted F2 seeds and it reverted to its parents in random. Or if seeds were saved from an OP, was cross pollinated. Now the question is why you are getting 2 shapes on one plant ? It has to do with the genes domination. JMO

Sey

Some varieties probably have shape variation naturally. For example, Prue has shapes that range from more or less round all the way to long, paste-type shapes with a nipple, with most being in the blunt heart to heart shape.

How oblong were they? Just a little more oblong than you expected, or a full-on plum shape? It would be unusual if they switched from being full-on plum shapes to full-on globes, but since there seems to be an association with when the fruit was set (as opposed to oblong and round being interspersed throughout the plant's development), I would tend to think that it was an environmental variable that affected how the fruit developed or the expression of a gene related to shape.

@gorbelly They started out a full-on plum shape, and are now quite round. It seems like I have a lot of plum-shaped tomatoes this year that were supposed to be other shapes (from at least three different sources). I should check on more of the others to see if they're now the right shapes, or if they're just regular crossed or mixed seeds.

Shule: It seems like I have a lot of plum-shaped tomatoes this year that were supposed to be other shapes (from least three different sources).

IIRC, herbicide damage can affect fruit shape. It's possible, if this is a general trend you're seeing in a bunch of your tomatoes, that they had a light touch of herbicide exposure at some point. Some viruses can also elongate fruit, but you'd probably notice a lot of other symptoms.

That's good to know. Soil in a path near those tomatoes used to have herbicide applied to it yearly years ago (glyphosate), and although it's not supposed to linger in the soil, I do have some tomatoes near that path that look to have received some herbicide with the transplant (we haven't used any anywhere this year), and are stunted. It wasn't my choice to apply the herbicide, though, and it's been a while since it's been used (supposedly a couple years).

It could be lingering herbicide causing the shape of that plant, although that plant is healthy and not stunted. Most of the other varieties are much further away from the path.

Aunt Molly's ground cherry sure doesn't mind growing on that path, though. Either does a multiflora cherry tomato that was supposed to be Jim Dandy (it looks more like Barry's Crazy Cherry or Ildi). Ovita sure didn't like it (it never matured, and the leaves looked to be dealing with herbicide; good thing I grew another Ovita in a container; it tastes great). Maglia Rosa had some damage on that path, but it still grew great-tasting fruit (which I used mostly for seeds). Ethel Watkins Best was stunted. Caro Rich was pretty damaged and stunted growing near the path. Coldset didn't prosper there, and eventually got a fungal infection. It produced one small orange tomato (I thought it was an accidental cross or mixed up seed, but maybe herbicide can affect color, too).

Anyway, next year I think I'll just put ground cherries all along that path.

Glyphosate shouldn't be lingering that long.

If the fruit shape issue was caused by herbicide, it would have been a very small amount or you would have noticed other damage. So it could easily be volatilized and blowing in from a neighboring property, in the air, etc. Tomatoes, for whatever reason, seem to be especially sensitive to a certain class of herbicides.

If you have a bunch of plants showing stunting in addition to fruit shape abnormalities, you could also be dealing with a virus (there is a lot of overlap between virus and herbicide symptoms). Scroll to the bottom of this page to see how CMV changed tomato fruit shape. The thing that makes me think it's not a virus is that your unnamed special sweet tomato grew out of the odd fruit shape, and plants can grow past herbicide damage but cannot grow out of a virus.

I have the same issue on my early girl tomatoes. At least that's what the plant was labeled. I have both plum or paste tomatoes and round tomatoes on the same plant.

Here's a picture of what I'm talking about