In Support of Winge's Theory of “Hybridization Followed by Chromosome Doubling”

Abstract

Polyploidy-or chromosome doubling-plays a significant role in plant speciation and evolution. Much of the existing evidence indicates that fusion of unreduced (or 2n) gametes is the major pathway responsible for polyploid formation. In the early 1900s, a theory was put forward that the mechanism of "hybridization followed by chromosome doubling" would enable the survival and development of the hybrid zygote by providing each chromosome with a homolog with which to pair. However, to date there is only scant empirical evidence supporting this theory. In our previous study, interspecific-interploid crosses between the tetraploid Hylocereus megalanthus, as the female parent, and the diploid H. undatus, as the male parent, yielded only allopentaploids, allohexaploids, and 5x-and 6x-aneuploids instead of the expected allotriploids. No viable hybrids were obtained from the reciprocal cross. Since H. undatus underwent normal meiosis with regular pairing in the pollen mother cells and only reduced pollen grains were observed, the allohexaploids obtained supported the concept of "chromosome doubling." In this work, we report ploidy level, fruit morphology, and pollen viability and diameter in a group of putative hybrids obtained from an embryo rescue procedure following controlled H. megalanthus × H. undatus crosses, with the aim to elucidate, for the first time, the timing and developmental stage of the chromosome doubling. As in our previous report, no triploids were obtained, but tetraploids, pentaploids, hexaploids, and 5x-and 6xaneuploids were found in the regenerated plants. The tetraploids exhibited the morphological features of the maternal parent and could not be considered true hybrids. Based on our previous studies, we can assume that the pentaploids were a result of a fertilization event between one unreduced (2n) female gamete from the tetraploid H. megalanthus and a normal (n) haploid male gamete from H. undatus. All the allohexaploids obtained from the embryo rescue technique where those that regenerated from fertilized ovules 10 days after pollination (at the pro-embryo stage), showing that the chromosome doubling event occurred at a very early development stage, i.e., at the zygote stage or shortly after zygote formation. These allohexaploids thus constitute empirical evidence of "hybridization followed by chromosome doubling."

Genetic Variability in Balkan Paleoendemic Resurrection Plants Ramonda serbica and R. nathaliae Across Their Range and in the Zone of Sympatry

“…For example, they may simply have come from a cross between a tetraploid (of whatever type) and a hexaploid, or even from the union of an unreduced 2 x gamete from R. nathaliae and a 3 x gamete from a hexaploid. These pentaploid individuals could also play a role as mediators of gene flow, but without forming an independent evolutionary unit ( Tel-Zur et al, 2020 ; Peskoller et al, 2021 ).…”

Breeding an underutilized fruit crop: a long-term program for Hylocereus

“…Previous work indicating the presence of unreduced gametes in H. megalanthus [ 22 ] suggested that the pentaploid hybrids probably originated from a fertilization event between an unreduced (4 n ) egg cell [ 22 , 34 ] from H. megalanthus and a normal haploid ( n ) pollen grain from the diploid paternal species [ 22 ]. The origin of the hexaploid hybrids was further studied using hybrids from an embryo rescue procedure following the crossing of H. megalanthus with H. undatus [ 35 ]. The findings indicated that these hexaploid hybrids were a result of hybridization followed by chromosome doubling at the very early stages of embryo development; see below the section, Embryo rescue following homoploid and interploid interspecific crosses.…”

“…The resulting putative hybrids showed triploid, tetraploid and higher ploidy levels, and fAFLP proved their hybrid origin [ 64 ]. Fruit morphology of the true hybrids combined the traits of the two parents, but fruit weight was statistically significantly lower than that of the parents, with the hexaploid hybrids bearing smaller fruit than the pentaploid hybrids [ 35 ].…”

“…A group of putative hybrids produced by the embryo rescue technique for the H. megalanthus × H. undatus cross were further studied with the aim of isolating triploid (instead of pentaploid, hexaploid, and 5 x and 6- x aneuploid [ 35 ]) hybrids that had not been obtained in our early work from germinated viable seeds following the same cross [ 15 , 22 ]. Contrary to expectations, embryos isolated 10 DAP (at the pro-embryo stage) by the embryo rescue technique did not yield triploid hybrids, but rather, once again, pentaploids, hexaploids, and 5 x and 6- x aneuploids [ 35 ], as were obtained from viable seeds, as described above.…”

“…A group of putative hybrids produced by the embryo rescue technique for the H. megalanthus × H. undatus cross were further studied with the aim of isolating triploid (instead of pentaploid, hexaploid, and 5 x and 6- x aneuploid [ 35 ]) hybrids that had not been obtained in our early work from germinated viable seeds following the same cross [ 15 , 22 ]. Contrary to expectations, embryos isolated 10 DAP (at the pro-embryo stage) by the embryo rescue technique did not yield triploid hybrids, but rather, once again, pentaploids, hexaploids, and 5 x and 6- x aneuploids [ 35 ], as were obtained from viable seeds, as described above. The pentaploids were probably a result of a fertilization event between an unreduced (2 n ) female gamete from H. megalanthus and a normal reduced ( n ) gamete from H. undatus , as was obtained following interspecific crosses (see above, the section First stages of breeding – interspecific homoploid and interploid hybridizations ).…”