GENOME COMPOSITION IN VENEZUELAN SPINY-RATS OF THE GENUS PROECHIMYS (RODENTIA, ECHIMYIDAE) .1. GENOME SIZE, C-HETEROCHROMATIN AND REPETITIVE DNAS IN-SITU HYBRIDIZATION PATTERNS

The genome sizes of the Venezuelan spiny-rats Proechimys guairae guair ae (2n = 48) and P. trinitatis (2n = 62) were evaluated and proved to be 12.5 +/- 0.5 pg and 12.6 +/- 0.3 pg respectively, the highest so fa r recorded among mammals; also the C-heterochromatin (32.7%, Coefficie nt of Variation [CV] 3.8 and 35.8%, CV 4.4) and GC (44.2%, CV 2.7 and 43.6%, CV 2.9) contents are very high. Highly repetitive (rep) DNAs we re isolated from restriction enzyme digested genomic DNAs of both spec ies. The intra-and inter-specific chromosomal allocations of these rep DNAs were analyzed by direct and cross-hybridizations. Results show t hat the two genomes harbour several rep DNA families which show both s pecies-specificity and interspecific relatedness in their in situ hybr idization patterns. The rep DNA families show an equilocal distributio n at both the pericentromeric areas of all chromosomes and in the whol e arms of two pairs of the uniarmed group, suggesting co-evolution of the rep DNAs. P. g. guairae BamHI digested DNA, when cloned and sequen ced, proved to consist of a long ''composite'' unit (1,239 bp) contain ing two copies of each of 75-bp and 110-bp internal subrepeats. Karyot ype restructuring between P. g. guairae and P. trinitatis, mainly due to Robertsonian changes, was accompanied by slight intra- and intergen omic movements of the putative satellite DNA families within stable ge nome sizes and C-heterochromatin contents. We discuss the findings obt ained in Proechimys in the light of those regarding the kangaroo rat, the pocket gopher and the house mouse; they support the idea that kary otype restructuring could be the expression of molecular driven events of rep DNA amplification and homogenisation through non-homologous ch romosomes.