Are there male and female watermelon fruits?

Question

This article, as well as folk wisdom, suggests buying "female" watermelons since they are sweeter and have less seeds.

"Female" species are said to have a larger black spot on the side opposite to the stem.

Wikipedia says that there are male and female watermelon flowers, of which only female ones produce the fruit, but I believe this does not apply to the fruit itself.

So the questions are: do the watermelons (fruits) with different black spot sizes really differ in sex (in whatever way), or it's just a metaphor helping to memorize which is which?

Does size of the black spot correlate with sweetness and amount of seeds?

Answer 1

Are there male and female watermelon fruits?

The claim

The referenced article claims that there exist female and male watermelons.

I think there are two ways of interpreting the claim of the existence of a male watermelon fruit.

1. male watermelon fruits are any watermelon fruits formed by male watermelon plants or by the male reproductive units of watermelon plants.

2. some watermelon fruits contain seeds that all grow into male watermelon plants

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Refutation

1: Fruit of male

"In botany, a fruit is a part of a flowering plant that derives from specific tissues of the flower, mainly one or more ovaries." - Wikipedia

"n. pl. fruit or fruits 1. a. The ripened ovary or ovaries of a seed-bearing plant" - The Free Dictionary

"Fruits are produced only by flowering plants (angiosperms). Following pollination of the flower, the fertilized ovules develop into seeds while the surrounding ovary wall forms the fruit tissue, or pericarp." - Biology encyclopedia

By definition, males don't have ovaries.

Botanically speaking, a "male fruit" is a contradiction in terms

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2: Fruit is male.

Plant sexuality is complicated.

In a few species, individual plants are either wholly male or female. In this case I believe the male plants do not produce fruit.

In many common plants, the individual flowers may be male or female (on the same plant) but the plant from which they emerge can not be said to be either.

Another way of classifying a fruit as "male" would be if the fruit were produced on a female plant but contained a single seed that would grow into a male plant. Since individual plants of some species can switch sexuality over time, the sex of an individual plant might not be wholly (or even partly) determined by the genetics of the seed from which it grew but by environmental influences on the growing plant.

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Update,

As Quassnoi commented, Water meolons are monoecious - a single plant bears both male and female reproductive units. Therefore the plant itself cannot be considered either female or male. Therefore nether can the seed from which it grows or the fruit that contained that seed.

"The watermelon plant is a slender, sprawling, slightly hairy, monoecious annual." - Dewey M. Caron, Univ. of Delaware

Answer 2

I believe they were talking about stenospermocarpy. Stenospermocarpic watermelons (or any other plants) pollinate properly, but the embryo aborts at a very young age. This causes a seedless fruit, which also tends to be sweeter.

Seedless banana and watermelon fruits are produced on triploid plants, whose three sets of chromosomes prevent meiosis from taking place and thus the plants cannot produce fertile gametes. Such plants can arise by spontaneous mutation or by hybridization between diploid and tetraploid individuals of the same or different species.

(reference)

So these watermelon plants don't produce male flowers, and it is necessary to plant a regular variety nearby, to pollinate the flowers. This could be why they were called female watermelons. But the male flowers never set fruit anyway, so all watermelons are from female flowers.

As for the black spot, it has more to do with variety than anything else, and though it will vary with the individual, I can speculate that the folk wisdom you mentioned was noting that the varieties that are seedless tended to have a larger black spot than the others, but I haven't noticed this myself.