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I imagined a trioecious angiosperm species descended from common cocoa trees (Theobroma cacao). Trioecious = three sexes (male, female, and simultaneous hermaphrodite).

They use the ZW sex-determination system (like Amborella trichopoda, Virginian strawberries, pistachios, Schistosoma, woodlice, lepidopterans, amphioxi, American eels, tilapias, giant salamanders, sirens, axolotls, cane toads, lacertids, anguimorphs, and birds). ZW = the default state is to develop as male, and the mother is the parent responsible for determining the sex of the offspring. In other words, male is the homogametic sex, and female is the heterogametic sex. When a female of a species that uses the ZW sex-determination system reproduces asexually (google parthenogenesis), they only have sons rather than daughters (because at least a Z chromosome is necessary to live, and having two W chromosomes or just one is a lethal condition).

However, this species is tetraploid rather than diploid.

So, I wonder how sex chromosome inheritance would be in a trioecious species that uses the ZW sex-determination system.

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  • $\begingroup$ I found a website you might be interested in, scroll down to the middle section for zw... $\endgroup$
    – user6760
    Commented Jan 10 at 6:52
  • $\begingroup$ For those readers who might be confused: (1) Theobroma cacao is monoecious, that is, it has perfect flowers, not dioecious or trioecious. (2) T. cacao is diploid (2 n = 10), not tetraploid. (3) Whatever the mysterious trees in the question are, they are definitely not T. cacao. $\endgroup$
    – AlexP
    Commented Jan 10 at 7:20
  • $\begingroup$ Data conflict detected. If ZW is a female, and it reproduces asexually, it cannot produce only males, as offspring would be ZW instead of ZZ. $\endgroup$
    – Vesper
    Commented Jan 11 at 7:45

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The article Preferential Pairing of Sex Chromosomes in the Tetraploid Silkworms (Bombyx Mori) shows that in silkworms that have been induced to be tetraploid (they are normally diploid), there are a number of possible pairings of the Z and W sex chromosomes, but that like-with-like pairing is much more common:

In the oocytes of the silkworm, Bombyx mori, only the sex chromosome bivalent (ZW) can be identified among the 28 bivalents, when we use the females of the sex-limited yellow cocoon (Sy) strain, in which the W carries a long fragment of the second chromosome. Autotetraploid silkwonns were induced from Sy females, in order to investigate whether preferential pairing between Z and Z as well as W and W occurs in the 4n oocytes. Two types of pairing patterns, (ZZ + WW) and (ZW + ZW), were observed in the oocytes, and the ratio of the pairing patterns, (ZZ + WW): (ZW + ZW), was 3421 in Sy4n (Sy9 x Sy$), while 16:2 in Syr4n (Sy9 x re93). Sex chromosome formulae of the eggs formed by the tetraploid can he confirmed when tetraploid females are crossed with sch males (sex-linked chocolate larva). The triploid offspring from Syr4n females mated with sch males, showed ZW gametes and ZZ gemetes produced in a 22:l ratio, which coincides with the ratio calculated from the pairing pattern ratio, 16:2. The results indicated preferential pairing of Z with Z and of W with W in tetraploid oocytes.

While this study was performed in insects rather than trees, the biology of cellular division and gamete production is common across the animal and plant kingdoms, and we could expect similar results in a plant species.

Since we don't know how the OP's proposed hermaphroditic sex occurs in this tetraploid species, it would be up to the OP to determine how a non-standard mixture of Z and W chromosomes would result in this condition. However, one possibility is detailed below:

In the (diploid) ZW sex-determination system, males possess two Z chromosomes, while females possess a Z and a W chromosome. Like the XY chromosome system, the ZW system also results in offspring being 50% male and 50% female. So far, there is no clear way to have trioecy and a hermaphrodite sex.

However, if we have tetraploidy (having four copies of the genome, and not just two as in diploidy), we no longer have the possibility of only ZZ or ZW, we have ZZZZ, ZZZW, ZZWW and ZWWW, with WWWW being lethal.

If we consider that ZZZZ is by necessity male, the presence of one or more W-chromosomes should make the offspring female or hermaphrodite. Since the study above shows that in meiosis, like pairs with like much more often than like with unlike (i.e. ZZ or WW occurs much more frequently than ZW), we can expect the majority of offspring to be ZZZZ or ZZWW.

I am arbitrarily stating the frequency of a ZW paring in meiosis where a ZZ or WW pairing is possible to be 1%, though it need not be exactly that frequency:

ZZZZ x ZZWW -> 49% ZZZZ, 49% ZZWW, 2% ZZZW

If we were to say that ZZZW led to hermaphroditic offspring, this would work. If we cross a hermaphrodite with a male or hermaphrodite respectively, we would get:

ZZZW x ZZZZ -> 50% ZZZZ, 50% ZZZW

i.e. 50% males, 50% hermaphrodite.

ZZZW x ZZZW -> 25% ZZZZ, 50% ZZZW, 25% ZZWW

i.e. 25% males, 50% hermaphrodite, 25% females.

However, if we cross a hermaphrodite with a female, we get:

ZZZW x ZZWW -> 24% ZZZZ, 24% ZZWW, 24% ZWZZ, 24% ZWWW, 2% ZZZW, 2% ZWZW

-> 24% ZZZZ, 26% ZZWW, 26% ZZZW, 24% ZWWW.

That is, less than a quarter Males, more than a quarter females, more than a quarter hermaphrodites, and less than a quarter... something else, ZWWW.

What is this ZWWW genotype? Since ZZZZ is male, ZZZW is hermaphrodite and ZZWW is female, is ZWWW some sort of super-female?

We have two options. The simplest is to say that ZWWW is lethal, so crosses between hermaphrodites and females are less fertile. The alternative is that we have another set of crosses, super-female with the other sexes:

ZWWW x ZZZZ -> 50% ZZZW, 50% ZZWW

i.e. 50% hermaphrodite, 50% female.

ZWWW x ZZZW -> 25% ZWZZ, 25% ZWZW, 25% WWZZ, 25% WWZW

-> 25% ZZZW, 50% ZZWW, 25% ZWWW

i.e. 25% hermaphrodite, 50% Female, 25% super-female.

However ZWWW super-female x ZZWW female is not possible.

So, it is possible to have trioecy if the genetic combinations ZZZZ are male, ZZZW are hermaphrodite, ZZWW are female and ZWWW is lethal.

Alternatively, we would have quadroecy... unless ZZWW females and ZWWW super-females were phenotypically identical, in which case the species would appear trioecious.

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    $\begingroup$ Gamete production is not homologous in plants and animals. Plants and animals have acquired sexual reproduction separately. Their most recent common ancestor did not reproduce sexually. $\endgroup$
    – AlexP
    Commented Jan 10 at 7:14
  • $\begingroup$ @AlexP from en.wikipedia.org/wiki/Meiosis, "Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi." $\endgroup$
    – Monty Wild
    Commented Jan 10 at 7:46
  • $\begingroup$ Duh, if they produce gametes then they must have a some form of a halving cell division. (The Greek word meiōsis means simply "diminution".) (In animals, the meiotic division produces gametes. Not so in plants, where the meiotic division produces haploid spores, which then undergo few or many cell divisions depending on what kind of plant we are speaking about, and only afterwards the haploid gametophyte produces gametes by ordinary mitotic cell division.) $\endgroup$
    – AlexP
    Commented Jan 10 at 9:03
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    $\begingroup$ Why are we down voting this? The goal is to help the OP rationalize a worldbuilding rule... not write a textbook on the science of the OP's world (no matter the hard-science tag). Even if the answer isn't 100% perfect science here on Earth, does it not answer the OP's question? $\endgroup$
    – JBH
    Commented Jan 17 at 18:37

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