The method(s) used for fixation of heterosis are

The method(s) used for the fixation of heterosis are Apomixis, Polyploid, and Asexual reproduction.
Apomixis is asexual reproduction without fertilization. Replacement of the seed by a plantlet or replacement of the flower by bulbils were categorized as types of apomixis.
Polyploidy is a condition in which the cells of an organism have more than one pair of (homologous) chromosomes.
Asexual reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant.
And lastly, we know that Heterosis refers to the phenomenon that progeny of diverse varieties of a species or crosses between species exhibit greater biomass, speed of development, and fertility than both parents.