A CENTRAL paradigm in evolutionary biology is that sexual reproduction is advantageous over asexuality(1-5). One of the long-term disadvantages asexual forms have to face is Muller’s ratchet(6). In the absence of recombination, theoretically no genotype can ever produce offspring with fewer mutations than its own load, The accumulation of deleterious mutations and gene combinations that cannot be purged should lead to extinction of parthenogenetic forms within 10(4)-10(5) generations(7,8). Evidence is accumulating, however, that some of these might have survived for such periods or even longer(9-14). In the Amazon Molly fish Poecilia formosa we have detected a process that appears to compensate for disadvantages of asexuality, namely incorporation of subgenomic amounts of DNA from a bisexual host species by microchromosomes.