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Wednesday, May 25, 2011

Crossover – Some possible events in my fishroom… ~ Updated 6.27.11 ~

Crossover – Some possible events in my fishroom…

© Alan S. Bias
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Just what is Crossover?  Or more precisely what is it not?  It is not a mutation, nor does it result in expected phenotype when breeding for specific Y-linked genotype(s).  This event occurs infrequently prior to  the process of fertilization, during meiosis, when genes cross over between chromosomes.  It is most noticeable in events that involve crossover between X & Y chromosomes.  To confuse things even further many times the event is not noticed in the F1, but the F2 offspring.  Ever wonder why pattern is absent, or that less colorful individual appears in a litter of otherwise normal looking siblings and you are reasonably sure he is not a mule (XX male or XXY male)?  In some strains crossover has the potential to go unnoticed by breeders.  Particularly, those in which similar sex linked traits (resulting from single gene or gene complexes) are commonly found on both the X & Y within a strain. 

Crossover in guppies is a concept that has the potential for broad misuse to easily explain events that are otherwise explainable with a better understanding of genetics.  Until recently guppy breeders found little documentation on crossover and knew little to what extent it might influence phenotype.  Yet, we often wondered why an occasional male did not express the same characteristics as his siblings?  We are taught that mutation only happens at rates of less than 1 in > a million.  So, that effectively rules out mutation in these fairly frequent oddities in the fishroom.   

I first took notice of seemingly unexplainable events in my fishroom back in the mid 1980’s shortly after starting to work with Vienna Emerald Green (VEG) Swordtails.  Rather than deciphering it in terms of what happened, I tried to look at things from the standpoint of what did not.  With my fishroom at the time geared more towards showing than genetic exploration, more often than not, such an individual was simply culled as a flaw in the program.  These days I am more inclined to retain oddballs, watch them grow out, and space providing do some test breeding.

Mixed Swordtails in a Community Tank Setting
In Swordtails genotype for color / pattern and sword shape are often found in a related gene complexes.  So it is fairly safe to assume the genes (alleles) reside very close on the same chromosome.  How do I know this?  In part because of the resulting phenotype when suspected crossover occurs.  It was not uncommon, every so often, for a VEG lower male to have nearly perfect pattern and an absolutely clear round tail.  No colored rays and no visible extension.  So how can I consider this as a potential crossover event?  In my Vienna Lower Swords it is possible for females to pass extension and color genetics, but not an X-linked lower swordtail gene(s).  So what is extension?  Just that, the ability to enhance the length of a son’s caudal &dorsal.

Typically, in Vienna Lower Swords both the sword and pattern are Y-linked.  These infrequent clear round tailed individuals have an Emerald Green peduncle and body color, but sometimes lack the “undulating / swirling” pattern needed to be considered a VEG. So at best they might be called of Vienna Type.  They lack the expected Y-linked phenotype for swords, color / pattern, and instead express two X-linked traits.  Those being a clear round tail and color only.  Strangely, I have no recollection of a similar event with Sauer’s type IFGA lower swords.
Vienna Type Male Lacking Sword

Another suspect event that occurred involved Panda Moscow Roundtails.  Every so often a male or two would appear in a litter that lacked expected Y-linked expression of both Moscow pattern and colored roundtail.  Instead these males were nearly devoid of color, less a light iridescent blue cast of spotting, and carried a long narrow caudal.  Much reminiscent of the scarf tails and ribbon tails I bred in the 1970’s as a teenager.  At first I thought this phenotype simply a homozygous expression of the Pink (pk) gene.  If a possible explanation, than pink must be in complex with the new tail shape?  I eventually leaned toward a crossover event after limited test breeding back to “pink” Panda females failed to produce in type any of these males in the F1.

Normal Snakeskin Top Swords

In the last year three distinct events have occurred.  The first involved Snakeskin Top Swords, the second Schimmelpennig Platinum Double Swords in conjunction with Snakeskin Top Sword and the third again Schimm. Platinum Double Swords.  I have been working with these snakeskins for about a year now.  Maintaining in two lines; a pure gold and a mixed grey / gold (blond).  During this time I have run numerous crosses, in both directions, with my other strains of swordtails.  For the most part this well established strain appears Y-linked for both a varied snakeskin gene complex, color, and top swords.

Snakeskin Top Sword * Snakeskin Top Sword siblings with crossover male on right

Too my surprise two males of the last litter lacked both snakeskin pattern and top swords.  As you can see they are clearly wild-type in coloration and finnage.  The caudal is a clear round tail with no visible extension.  As this phenotype is not found in my stocks, and individuals where housed on a stand alone rack consisting solely of related lines, it may be safe to assume these two males are the result of a crossover event.  You will also notice lacking the physiological need to expend resources in growing swords, the dorsals in these males grew out very quickly compared to topsword brothers.

Snakeskin Top Sword * Snakeskin Top Sword siblings with crossover males below

Now the next event is a bit more perplexing.  Over the last decade I have made dozens of pure and cross breedings with Schimmelpennig Platinum Double Swords as the sire.  Frequently with my Vienna Lower Swords in search of a crossover event  involving Y-linked Platinum to X-link.  I routinely breed these Schim. sired females to my Y-linked lower males in hope of creating a Schim. Platinum Lower with fruitless results.  So far the Y-linked  Schim. Platinum gene complex is just too closely linked to a Y- Double Sword gene(s).   (Note:  Keep in mind many double swords result from a Y-link lower and X-link top in combination.  Schim. Platinum are also found in conjunction with lyretail, roundtail, pintail and speartail.  Though, platinum speartail and pintails normally result from Y-link platinum and X-link caudal shape).   

F1 Schim. Platinum Double * Lace Snakeskin Top Sword
Last winter I bred a multi-generation pure Grey Bodied Schim. Platinum Double with gold shoulders * Grey & Gold (Blond) Snakeskin Top Swords.  In part to identify if the latter strain was X-linked for snakeskin &/or top swords, and if so result in a Medusa / Galaxy.  The ½ dozen females I selected contained neither and all males colored similar to the above individuals.  Pretty much identical to their sire, less the addition of a vertical “mood” bar behind the gill plate and on the front shoulder.  This bar gets exceedingly dark during display.  In this case it may result from a snakeskin influence, but may not as the trait appears on occasion in my Schim. * Vienna breedings.  As a precaution I saved one last litter from one of the females.  The female had been left in the same tank in a breeder net with no further exposure to outside males.

F1 Schim. Platinum Double * Lace Snakeskin Top Sword

Her male offspring from this litter are shown above.  At first glance these males look to be Japan Blue.  Those showing green simply reflect the addition of yellow to the pattern.  Did the Top Sword female have an X-link for Japan Blue possibly masked in males by the snakeskin phenotype?  If they remain as is, I suspect these males are expressing the normal metal found as X-linked blue / green base body color found in the strain, less the Y-linked snakeskin overlay.   

Another type of metal?  While the color does extend forward, it is not the typical pattern found in Stoerzbach Metal Swords so I rule it out for the moment.  So, back to the primary issue - Where is the dominant Y-linked Platinum from the sire?  Remember Schim. Platinum males are a combination of platinum and Vienna genetics.  This points us back to a possible crossover event with the resulting phenotype likely an interaction of Emerald Green Iridescent (SmIr) & a X-linked base color.  Space permitting, I may test breed them at a later date. 

F1 Schim. Platinum Double * Lace Snakeskin Top Sword
   While the sheer number is suspect, these young males lend credence to a crossover event.  Multiple traits of the Schim. Platinum & Double Sword gene complex have been suppressed.   The resulting males exhibit non platinum pattern, non Vienna pattern, and clear roundtails or developing lower swords with as of yet no visible extension.  Dams from this cross were on an isolated rack with no known exposure to Japan Blue males.

F1 Schim. Platinum Double * Lace Snakeskin Top Sword

F2 Schimmelpennig Platinum
 sired males
~6.27.11~   Recently I noticed another  possible case of crossover, but have been waiting for the fish to color up.  In a breeding of  F2 Gold Schimmelpennig Platinum Doubles two males were lacking expected pattern.   Not a hint of platinum expression and clear round tails without swords.   Once again multiple traits of the Y-Linked  Platinum & Double Sword gene complex have been suppressed.   The resulting males exhibit non platinum pattern & extension with only partial Vienna pattern.  Within a few days of photographing the dorsals started to color from an X-link for color.

F2 Schimmelpennig Platinum sired males
While I am warming to the concept of crossover it is just not one I wholly embraced.  There are often far to many other genetic explanations for these "seemingly unknown" events.  If this breeding resulted from a multi-generation linebred strain crossoever would be more plausible.  However, the do not.  I frequently utilize my Vienna females in matings with Schimmelpennig males.  Lacking a controlled setting there is margin for error in this example and one should view with skepticism.

Recent publications have shown us crossover can potentially affect, but is not limited to, color – pattern – tail shape.  As a result many breeders quickly rule out the explainable event as crossover.  So are any of these events crossover, or can they too be explained by additional genetic principles?  


1 comment:

  1. Yet another very interesting article to add to the collection - Bill Galbally - UK.