ABSTRACT
We studied the evolution of RAE180 satellite DNA family in the North American endemic dioecious plant Rumex hastatulus. In this species, the Texas race is characterized by a single XX/XY sex chromosome system, whereas the North Carolina race has evolved a derived complex XX/XY(1)Y(2) sex chromosome system. RAE180 repeats were autosomic and poorly represented (2 × 10(-4)% of the genome) with no differences between individuals of different genders or different races of R. hastatulus. In fact, the sex chromosomes of the North Carolina race are still euchromatic, and they have not accumulated satellite DNA sequences, which contrasts with that occurring in the rest of dioecious XX/XY(1)Y(2) Rumex species. In R. hastatulus, we detected the existence of three RAE180 subfamilies. Notwithstanding, while in the Texas race the TX1/NC1 subfamily is the most frequent, the TX2/NC2 subfamily is the most abundant in the North Carolina race. Additionally, the third, less represented subfamily (TX3/NC3) appears currently as relict sequences in both genomes. A common feature of RAE180 satellite is the sudden replacement of one sequence variant by another in different species (or populations as in R. hastatulus races). Thus, the phylogenetic analysis of RAE180 repeats from six dioecious Rumex species supports the "library" hypothesis. According to this hypothesis, we assume that a set of divergent RAE180 variants were present in the ancestral genome of dioecious Rumex species, from which novel tandem arrays originated by the amplification of different variants in different lineages. Differential levels of RAE180 satellite DNA amplification in each lineage, at different evolutionary times, and in different chromosomal positions gave rise to differential patterns of sequence evolution.
