No abstract available

Hereditary melanoma in Xiphophorus hybrids canying the melanoma·induclng Tu-Sd locus is caused by transcriptional activation of the Xmrk gene that resides at the Tu·Sd locus and encodes a novel member of receptor tyrosine kinases (RTK). ln this study, a total of 17 hereditary melanomas from various hybrid genotypes harbouring 7 different Tu alleles were also found to aver-express the correspondlng Xmrlc alleles. The Ievei of over-expression correlated with the degree of malignancy of the melanoma. ln addition, Xsrc expression was high ln many malignant melanomas. Expression pattems and Ieveis of the Xiphophorus EGF-receptor gene (Xerb B), the c-myc (Xmyc), and the PDGF (Xsls) gene(s) were not intriguing. Transcription of the ras gene(s) may be correlated to secondary events of melanoma progression. Expression pattems of Xfms, the Xiphophorus CSF-1 receptor homologue, can be explained by different contents of infiltrating macrophages in the tumors. ln carcinogen-induced tumors includlng one melanoma no significant expression of the Xmrk oncogene could be detected. Xsrc expression, however, was strikingly high. This indicates that activation of oncogenes other than Xmrk ls instrumental in tumorigenesls of neoplasia of non-hereditary origin.

No abstract available.

Pseudotropheus hajomaylandi (loc. typ. Isle of Chisumulu, Lake Malawi) is described as a new species. It is compared with Ps. aurora, Ps. greshakei, Ps. livingstonii, Ps. lombardoi, and Ps. zebra. All these taxa, including Ps. hajomaylandi and Ps. heteropictus, are classified in the subgenus Maylandia.

The conspicuous colour sexual dimorphism of guppies has made them paradigmatic study objects for sex-linked traits and sex chromosome evolution. Both the X- and Y-chromosomes of the common guppy (Poecilia reticulata) are genetically active and homomorphic, with a large homologous part and a small sex specific region. This feature is considered to emulate the initial stage of sex chromosome evolution. A similar situation has been documented in the related Endler’s and Oropuche guppies (P. wingei, P. obscura) indicating a common origin of the Y in this group. A recent molecular study in the swamp guppy (Micropoecilia. picta) reported a low SNP density on the Y, indicating Y-chromosome deterioration. We performed a series of cytological studies on M. picta to show that the Y-chromosome is quite small compared to the X and has accumulated a high content of heterochromatin. Furthermore, the Y-chromosome stands out in displaying CpG clusters around the centromeric region. These cytological findings evidently illustrate that the Y-chromosome in M. picta is indeed highly degenerated. Immunostaining for SYCP3 and MLH1 in pachytene meiocytes revealed that a substantial part of the Y remains associated with the X. A specific MLH1 hotspot site was persistently marked at the distal end of the associated XY structure. These results unveil a landmark of a recombining pseudoautosomal region on the otherwise strongly degenerated Y chromosome of M. picta. Hormone treatments of females revealed that, unexpectedly, no sexually antagonistic color gene is Y-linked in M. picta. All these differences to the Poecilia group of guppies indicate that the trajectories associated with the evolution of sex chromosomes are not in parallel.