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Maculatus Hybrids
I've always had a preference for swordtails, but it’s easy to understand why the intense coloration and variety of spotting patterns found in platies make them extremely popular.
As I described in my Melanoma in Platies and Swordtails article (Livebearers #224 2014), crosses between certain black pigmented platies and swordtails often produce hybrid offspring that develop melanoma. Some of the hybrids maintain the platy phenotype and have reduced melanoma risk (image A below). Other hybrids (images B and C) have likely lost the platy tumor suppressor gene resulting in excessive proliferation of macromelanophores at the caudal peduncle and an increased likelihood of developing melanoma.
Birchmanni-Variatus Hybrids



























































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Dorsal Fin Modifier Gene?
In my article, Why It’s So Difficult to Obtain Great Hi-fins (Livebearers #197 2007) I wrote that hi-fin appearance is controlled by the expression of the dominant Simpson hi-fin gene (H) and a number of modifier genes that in conjunction with H regulate the number of rays, the length of the rays, and the extent of dorsal fin branching. Evidence for the existence of dorsal fin modifier genes is good, but circumstantial at best. No lowfin (lacking the H gene) fish expressing a dorsal fin modifier that enhances the appearance of the dorsal fin has been identified.   


When I bred the above variatus-swordtail hybrid males to low finned marigold swordtail females all of the hi-fin offspring had average quality dorsal fins. This is consistent with the notion that at least some of the genes (modifiers) required to produce the high quality, long flowing dorsal of the male parent are recessive and not expressed in the offspring (F1 generation). Interestingly, two of the presumptive lowfin male offspring had unusually shaped dorsal fins compared to normal as shown below.
The modified dorsal fin has a distinct shape. Overall, the dorsal fin rays are longer than normal (compare the upper images of the whole fish) and the 3rd and 4th rays are extended. Additionally, as the fish matures (see below) the posterior rays continue to grow and extend to the caudal fin.
Normal Dorsal
Modified Dorsal
The expression of a novel dorsal fin modifier gene may be responsible for the altered dorsal fin shape. Alternatively, these fish may express the H gene and just lack the modifiers needed to produce a standard hi-fin phenotype. Additional crossings with unrelated lowfins and hi-fins may help to distinguish between these possibilities.
One of my favorite female hifin swordtails.
From the Archives
White swordtails with red caudal fins are rare.
Variatus Hybrids