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In my futuristic world, there are two coelacanth species that are the daughter species from the famous West Indian Ocean coelacanth. One has the XY sex determination system (like humans, iguanas, salmons, sharks, hoverflies, cannabises, papayas, and gingkoes), the other has the ZW sex determination system (like swans, monitor lizards, axolotls, tilapias, morphos, strawberries, pistachios, and Amborella trichopoda). They can still interbreed and (surprise!) the resulting offspring is fertile (like a human that has a Cro-Magnon mother and a Neandertal father, a wolfdog, a beige bear/white and brown bear hybrid, or even a swine that has a wild boar father and a domestic pig mother). Naturally, there are four possible pairs of sex chromosomes (excluding cases of uniparental disomy) for the resulting offspring: WX (female), YZ (male), XZ (intersex with an ovary and a testicle that can only reproduce sexually), and WY (intersex with ovotestis that can reproduce asexually).

In real life, there is a wide variation of sex determination systems among the nine living classes of Euteleostomes. If mammals have the XY sex determination system or exceptionally XO, birds exclusively have the ZW sex determination system, crocodilians only utilise the environment-based sex determination system, amphibians utilise both ZW and XY (salamanders = mostly ZW, frogs = mostly XY, and caecilians = no preference), actinopterygians, lepidosaurians, and turtles utilise all of the three (actinopterygians = mostly XY, lepidosaurians = mostly ZW, and turtles = mostly ESD). However, no one knows exactly how do both coelacanths and lungfish determine sex.

So, I wonder if it is possible for two species from the same genus but with a different sex determination system to interbreed and the resulting offspring is fertile (or at least sub-fertile).

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    $\begingroup$ Given the absence of any known examples here on Earth, how are you expecting any answer to meet the [hard-science] tag requirement? Maybe think about changing to [science-based]? $\endgroup$ Dec 7, 2023 at 3:09
  • $\begingroup$ what does sub fertile mean? It's fertile with one but not the other? $\endgroup$
    – Kilisi
    Dec 7, 2023 at 3:20
  • $\begingroup$ Sheep and goat can produce offspring (stillborn) but offspring nonetheless despite both come from different genus $\endgroup$
    – user6760
    Dec 7, 2023 at 6:59
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    $\begingroup$ Do you know of any one genus of animals containing species with two different heterosome-based sex determination systems? $\endgroup$
    – AlexP
    Dec 7, 2023 at 9:16
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    $\begingroup$ As others said, Hard-Science doesn't really fit this question as there are no known examples of it actually happening as far as I am aware. I second this to being changed to Science-based. $\endgroup$ Dec 7, 2023 at 16:04

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It's possible, but it would result in more than 2 possible sexes. In 2015, the clawed frog Xenopus tropicalis was discovered to have 3 sets of sex chromosomes: Y, W, and Z. This results in 3 different types of males (YW, YZ, and ZZ) and 2 types of females (ZW and WW).

Whether two species, one with XY and the other with ZW, could successfully procreate, probably depends on the exact combination of the chromosomes. A male XY coelacanth and female ZW coelacanth would result in XZ, XW, YZ, and YW, whereas a female XX coelacanth and male ZZ coelacanth could only result in XZ. This first generation of hybrids would be 40% XZ, and 20% each of XW, YZ, and YW. We know from X. tropicalis that YW and YZ are males, and XW and XZ are probably female. This results in a 0.67:1 sex ratio (m:f).

The next generation would get even more complicated.

YW male and XW female: XY (♂), YW (♂), XW (♀), and WW (♀).

YW male and XZ female: XY (♂), YZ (♂), XW (♀), and ZW (♀).

YZ male and XW female: XY (♂), YW (♂), XZ (♀), and ZW (♀)

YZ male and XZ female: XY (♂), YZ (♂), XZ (♀), and ZZ (♂)

Generation 2 has a sex ratio of 1.29:1

It is 25% XY (♂), 12.5% YZ (♂), 12.5% YW (♂), 6.25% ZZ (♂), 12.5% XW (♀), 12.5% XZ (♀), 12.5% ZW, and 6.25% WW (♀)

After Generation 2, pretty much every combination is present and the sex ratio is ~1:1. How many of these would result in viable offspring, I'm not sure.

Edit: thank you @mammifereviolet4694 for bringing to my attention the differences in sex chromosomes among closely related species in the genus Bufo. The toads B. bufo and B. spinosus have ZW and XY sex chromosomes, respectively. Apparently, hybridization has actually been documented: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7523563/

<there's a lot going on here, if there are any inconsistencies, let me know so I can fix>

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