Ploidy-mediated reduced segregation facilitates fixation of heterozygosity in the aromatic grass, Cymbopogon martinii...

In most medicinal and aromatic plants, the vegetative tissue (e.g., roots, stems, leaves) is the source of the economic product. These plants are inherently heterozygous (natural allelic hybrids) and maintain their genetic makeup in nature by obligate vegetative propagation. Under seed cultivation, these plants incur population heterogeneity that reduces biomass and hampers product quality. Therefore, fixation of heterozygosity is vital for maintaining uniformity in quality of the economic product and quantity of biomass under seed cultivation. Although seed-grown clonal progenies identical to the mother plant can be obtained in certain plants that show an unusual breeding system called apomixis, such a breeding system is rare in medicinal and aromatic plants of economic value. Here we show an effective experimental strategy based on a polyploid model that facilitates fixation of heterozygosity in obligate asexual species owing to tetrasomic inheritance and low segregation in C(1) progenies from high-fertility C(0) autopolyploids. Using an obligate asexual species of aromatic grass-Cymbopogon martinii, we demonstrated that progenitor diploids with distal chiasma localization and low chiasmate association in meiosis, when changed into tetraploids, entail high gametic/seed fertility reflected in high bivalent pairing and balanced anaphase segregation. Their seed progenies evince crop homogeneity owing to reduced segregation, indicating fixation of heterozygosity present in the source diploids. Because C. martinii could be maintained through obligate vegetative propagation, here is a unique opportunity to utilize the polyploid advantage through C(1) seed progenies for commercial cultivation, as well as maintenance of original C(0) stock for raising seeds without losing polyploid heterosis normally threatened in subsequent segregating progenies on account of aneuploidy and gametic instability.