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I found it very surprising, due to: > American paddlefish (Polyodon spathula), which are diploid, have 60 pairs of chromosomes, while Russian sturgeons (Acipen

by isp 3y ago

I found it very surprising, due to: > American paddlefish (Polyodon spathula), which are diploid, have 60 pairs of chromosomes, while Russian sturgeons (Acipenser gueldenstaedtii), which are tetraploid, have four sets totaling about 250 chromosomes (the chromosomes are so numerous and some are so small that it’s hard to count them reliably). Nearly 200 million years of independent evolution should have seeded the two species’ DNA with countless genetic mismatches and incompatibilities — from missing and added genes, to rearrangements and relocations of genes, to mutational tweaks to gene expression. It seemed certain that hybrid cells would struggle to figure out how to line up their chromosomes during cell division and what genes to turn on or off. As the researchers said: > “We never wanted to play around with hybridization,” said Mozsár. “It was just a negative control, which found, somehow, a way to live.”

adrian_b 3y ago
Such differences in the number and organization of chromosomes are the reason why the hybrids of distantly related species are usually sterile. Only when gametes are produced by the hybrids for their reproduction, during the meiosis the homologous chromosomes must pair and in such hybrids this fails. On the other hand, when the first hybrid cell is formed by the fusion of gametes, and also during its later divisions, which will grow the hybrid body, the number and organization of the chromosomes matters very little. What matters is just the amount of differences in individual genes and in their regulation, which should be small enough so that their functions remain compatible, which must be true in this case. It is normal in animals for the number of chromosomes to fluctuate due to fusions or fissions and for the order of the genes to be modified, but such changes do not modify the complete set of genes or their functions and these changes may happen without any changes in the appearance of the animal.

by isp 3y ago

adrian_b 3y ago

Such differences in the number and organization of chromosomes are the reason why the hybrids of distantly related species are usually sterile. Only when gametes are produced by the hybrids for their reproduction, during the meiosis the homologous chromosomes must pair and in such hybrids this fails. On the other hand, when the first hybrid cell is formed by the fusion of gametes, and also during its later divisions, which will grow the hybrid body, the number and organization of the chromosomes matters very little. What matters is just the amount of differences in individual genes and in their regulation, which should be small enough so that their functions remain compatible, which must be true in this case. It is normal in animals for the number of chromosomes to fluctuate due to fusions or fissions and for the order of the genes to be modified, but such changes do not modify the complete set of genes or their functions and these changes may happen without any changes in the appearance of the animal.