then cloud and deciduous forests reminiscent of the Smok- ies, to the tropical lowlands at Tamazunchale at about 300 m.  The  road  crosses  not  a  single  stream  along  its  entire length; it follows closely the top of the long divide between the Rio Amajaque to the east and the Rio Moctezuma to the west  because  this  minimizes  the  ups  and  downs  across mountain  ranges.  In  several  places  the  road  straddles  the very  top  of  the  narrow  divide.  There  were  long  stretches where  the  road  was  still  under  construction.  Not  a  single piece  of  mechanical  equipment  was  used,  only  dynamite, crowbars, and pickaxes. The expedition reached the Rio Moctezuma at Tama- zunchale, but was unable to fish. The main river is too deep and swift as it comes out of a gorge, and Myron could not locate  any  side  channels  or  small  tributary  streams.  They pushed  on  another  30  km  and  were  stopped  at  the  Rio Axtia. Men used mules to pull the cars through this river, but it was late in the day and the men had stopped working. Seining in the river near their campsite yielded Montezuma swordtails,  X.  cortezi,  and  beautiful  X.  variatus.  Many  of these fish were preserved. The next morning they rushed to the railroad station in Ciudad Valles, 75 miles to the north, to  claim  their  cans,  which  had  been  waiting  for  them  for well over 6 weeks. The expedition returned to the Rio Axtla and filled 12 cans with 20 fish each. The next day the fish were successfully shipped by railroad to New Orleans. Far- ther north other successful collections of X. xiphidium were made.  The  expedition  returned  to  Cornell  by  the  end  of May.  Sorting  through  the  preserved  collections,  Gordon was eventually left with a single fish that he could not iden- tify; it turned out to be the first pygmy swordtail. As  early  as  1931,  Gordon  had  been  wondering  how many genes were involved in the exaggerated expression of the macromelanophore genes in hybrids with helleri. But he considered  it  an  almost  impossible  task  to  determine: “While it is probable that many modifying factors are op- erating in Xiphophorus which influence the degree of mela- nosis,  it  is  hopeless  to  establish  the  exact  number  without increased facilities” (Gordon, 1931). Like every investigator after  him,  Gordon  needed  more  tanks!  Between  1932  and 1938 Gordon made good use of the fishes he had brought back; he crossed X. maculatus from the Rio Papaloapan with 3 other species, X. variatus, X. xiphidium, and X. couchianus. In  each  of  these  crosses  the  expression  of  the  macromela- nophore gene of X. maculatus was enhanced, but to differ- ent  degrees.  The  maculatus  ×  couchianus  hybrids  were  es- pecially  interesting  because  they  exhibited  the  neoplastic disease  at  birth.  Thus  the  modifier  genes  of  these  species could  not  be  the  same,  but  the  question  of  whether  they were  different  alleles  at  the  same  locus,  or  different  genes altogether, was not addressed (Gordon and Smith, 1938a). These  experiments  also  established  that  the  occurrence  of melanoma  and  pigment  cell  abnormalities  was  not  re- stricted  to  maculatus  ×  helleri  hybrids,  but  was  a  general phenomenon in  Xiphophorus. Gordon (1937b) employed the concept of multiple fac- tors to explain the formation of melanoma in the hybrids. Closely following Kosswig (1931) in Germany, he hypoth- esized  that  2,  but  perhaps  many  more,  modifier  loci  were contributed  by  X.  helleri.  But  it  is  not  clear  from  his  data how he arrived at this conclusion, and a year later Gordon and  Smith  (1938a)  stated  the  the  2-factor  hypothesis  was really  a  somewhat  arbitrary  construct.  Later  in  the  same year,  Gordon  and  Smith  (1938b)  wrote  that  the  swordtail contributes apparently more than one dominant modifier. Gordon thought that the two species, maculatus and helleri, possessed  different  alleles  at  the  2  loci,  aa  bb  and  AA  BB, respectively.  Those  hybrids  exhibiting  the  most  advanced state of melanosis were thought to have inherited, in addi- tion to the macromelanophore factor, all 4 dominant modi- fiers. But there was no independent confirmation that this interpretation was correct. Detailed  histological  descriptions  of  the  abnormal macromelanophore  patterns  were  provided  by  Reed  and Gordon (1931) and Gordon and Smith (1938b), who clas- sified  the  melanotic  overgrowths  into  3  stages.  The  first stage  is  characterized  by  a  macromelanophore  hyperplasia in the corium, the second stage by an invasion of the mus- cular  tissues  by  macromelanophores  along  the  myoseptae with some tissue destruction of fin rays and the soft tissue between  them,  and  the  third  stage  by  the  appearance  of invasive spindle-shaped cells, significant tissue destruction, and  melanotic  overgrowth.  No  metastases  were  observed. They emphasized that there is no sharp separation between the 3 stages and that the development of the pigmentation can  stop  at  any  stage.  The  histological  description,  of course,  was  important  not  only  in  its  own  right,  but  also because it served to make the  Xiphophorus  melanoma sys- tem  highly  pertinent  to  the  medical  community  at  large. “The  spindle  cells  of  the  melanotic  overgrowths  in  hybrid fishes histologically resemble the cells of mammalian mela- nosarcoma. They also are infiltrative and destructive to ad- jacent  tissue”  (Gordon  and  Smith,  1938b).  This  was  fol- lowed 3 years later by a report of growing the Xiphophorus melanoma in tissue culture. “In its morphology and behav- ior, including its property of clasmatosis, it is identical with S10 Klaus D. Kallman Next >>