Nowadays, it's becoming a taboo to speak of a birth defect when we are speaking of hermaphrodites. But yes, hermaphrodites do occur in ovoviviparous livebearers but not in viviparous livebearers (as far as we know). But in ovoviviparous it's a bit hard to claim that when females change into males should be labeled as hermaphroditism. For there are females that do have male gonads but have a complete female phenotype and there are specimens that have both male and female features already ever since almost the beginning. So yes, there are livebearers that do have both male and female gonads. But in that case, they all will start off as being a female. From what we know is that specimens that seem to be female change into male are coming from an XX, WX, WY combination. And a combination with the Z sex chromosome seems to be making a sex change possible. In normal situations we only know XX as being female and XY as being male. But it's already known that that there are more combinations possible. Which is already a step forward to dive into the sex genetics. Those people who claim that sex change is not possible only believe that there is just an XX or XY combination possible. And that this is just an urban story. So, only male and female. It doesn't only concern fish but also other live forms. They're just lacking knowledge in the sex genetics.
Extern factors can trigger a female to become a male. With the exception of the Xiphophorus genus (where swordtails and platies belong to), it's very rare. But real hermaphrodites in ovoviviparous livebearers are both male and female at the same time and will show as adults real large specimens with an undeveloped gonopodium. Such specimens won't be able to reproduce themselves. And yes, they do show a gravid spot as well, because they've started off as being female. Once a gravid spot is present, it will remain visible. For the gravid spot is just a piece of skin that does have a lower density than other skin parts of the body. But don't confuse them with late males. Late males are initial males from the beginning. And late males never show a real gravid spot.
In swordtails and platies it's already been proven how it's possible that certain phenotypical females can change into male. Depending on wether it has got to do with swordtails or platies, the combination of alleles and sex chromosome combination is key to produce females that can change into male and wether they'll be able to fertilize other females or not. It's already been researched in other ovoviviparous livebearers as well but, they haven't cleared it yet how it actually works in other ovoviviparous livebearers. But they've observed that those sex changes do occur. And also with those other livebearers that had a sex change, started off as being female.
The story about swordtails and platies changing sex has already been mentioned by me. But eventhough both species are in the same genus, the sex change takes place in a different way in both species. Because sex change happens differently in swordtails in comparison to platies, researchers think that it's possible that sex change in other ovoviviparous livebearers could also differ. It's also a theory of researches that this phenomenon has got to with keeping the population going. And that it's probably a way of how Mother Nature works to let a species survive. It's not an uncommon thing. It's known that it does also happen in other living species on earth.
I also have to mention that all that's been mentioned overhere, wild species have been used and not fancy species for this research. This is why some fancy platy strains and fancy swordtail strains have a sex change situation that should fit the other species. For most nowadays fancy platy strains and fancy swordtail strains are results of crossing between those two species. Which makes a sex change in fancy platies and fancy swordtails doubtful wether it happened by a certain allele combination or a specific sex chromosome combination.
And again, I'd like to refer to a socalled pseudo gravid spot. A pseudo gravid spot only appears in certain male livebearer species. Most known are male platies. But there are also other livebearer species that can show this phenomenon. But such a pseudo gravid spot has got the same density like other skin parts. And such a pseudo gravid spot is a matter of having more melanophores developing on the same spot where a female has got her gravis pot. But once a pseudo gravid spot appears, it will never fade. Most of the time such a pseudo gravid spot in platies will occur when they're close to adulthood.
And the story about old females show male features....which features are shown in the finnage of the old female... Well, that has got to do with the MSX genes. Just like humans older female ovoviviparous livebearers, can stop being reproductive. When that happens, a hormonal change takes place which triggers the activity of the MSX genes in a higher activity. This results in male features in finnage (larger and/or elongated fins). This concerns all fins possible. But mostly seen in dorsal, caudal and anal fin. For the MSX genes are responsible for the morphological change in finnage. But it will remain a female specimen and has got nothing to do with turning into male.
But to answer the question: Can female guppies change into males? The answer will be: That's possible. But again, very rare...
