J. Amer. Soc. Hort. Sci. 120(5): 734-740. 1995.
Variation in randomly amplified DNA markers and storage root yield in 'Jewel' sweetpotato clones
Arthur Q. Villordon and Don R. La Bonte

Genetic variability within a clone alter favorable genetic combinations and contribute to cultivar decline in the highly heterozygous sweetpotato. Reduction of productivity in clones has been generally referred to as clonal degeneration in other crops (Simmonds 1979), more specifically as cultivar running-out in sweetpotato (Miller etal., 1959). Virus infection has also been identified as a factor in plant clonal degeneration (Richards, 1986). Specifically, Gooding (1964) cites virus as the cause of yield decline in a West Indian sweetpotato population. At the same time, Muller (1964) proposed that species without a sexual phase are not only impeded in evolution but also subject to genetic deterioration. He proposed a ratchet mechanism where mutational load can only increase compared with existing levels. This ratchet mechanism was suggested for haploid sexual organisms, but can also apply to diploid and polyploid organisms as long as these mutations are expressed in the heterozygous condition (Leslie and Vrijenhoek, 1980).

The net effect of Muller's ratchet in causing deterioration is a a function of genome (more loci to mutate) and population size (Maynard-Smith, 1978). The polyploid nature (2n = 6x = 90) of the sweetpotato can contribute to the unusually high rates of visible mutation in the crop. If a constant number of random mutations occur per generation in a haploid genome, then a triploid will experience three mutations for every two experienced by a diploid (Mogie, 1992). Thus, in the hexaploid sweetpotato, six mutations are theoretically possible compared to two in a diploid counterpart. At some point, the extra genome dosage may not sufficiently buffer additional deleterious mutations and also the complex interactions among mutant sites, wild type alleles, and the prevailing environment. This is synonymous to attaining a mutation/selection equilibrium in Muller's ratchet mechanism before mutational load increase. Presuming the ratchet mechanism operates, and assuming all deleterious mutations are expressed, then cultivar decline becomes a function of time.

The mode of propagation can also contribute to the crop's predisposition to genetic variability. In tropical propagation systems, the sweetpotato is virtually a perennial, where stem cuttings are collected from standing crops in a continuous planting procedure (Simmonds, 1976). In contrast, the crop is grown as an annual in the United States and in other subtropical production areas. In these areas, the sweetpotato sprout is the unit of propagation. These sprouts are derived from adventitious buds on storage roots. Adventitious buds may develop in callus, wound periderm, the vascular cambium or in anomalous cambia (Edmond and Ammerman, 1971; Fahn, 1982). Adventitious bud production, particularly from callus or anomalous cambia, originate from previously non-meristematic cells. This nonmeristematic origin can contribute to systematic variability in subsequent generations. For example, in vitro cultures established from nonmeristematic tissue in other plant species have a higher rate of variation than cultures from organized tissues (meristems) (D'Amato, 1985). Gould (1984) identified differences in the duration of various phases of cell cycles in meristematic and nonmeristematic dividing cells in tissue cultures of various plants. Such disturbances cause delay in DNA replication in heterochromatic regions and result in genetic variation (Lee and Phillips, 1988). Assuming absence of strictly regulated cell cycles, each round of sweetpotato clonal multiplication potentially introduces subtle variability that may accumulate over time. Hence, certain aspects of the sweetpotato propagation system may resemble somaclonal variation observed in in vitro systems.

Variation in yield. Differences in yield measurements suggest the range of mean productivity among clones. The absence of first order interactions for yield data suggests that the change of mean rankings across environments was not significant, and the differential response among genotypes, without rank change across environments, was also not significant. If root samples are accurate representations of each foundation program, then data can be taken to reflect the range of cultivar yield variability among clone sources 27% to 46%) across the sample environments. The differences in mean performance among clonal classes can be attributed to the following factors that act singly or jointly: I) random and systematic fixation of deleterious and beneficial mutations, 2) interaction between the environment and mutant and wild type alleles, and 3) temporal and spatial variation in size of the source foundation seed populations

Conclusions

At the phenotypic and genotypic level, our results suggest inherent variability still exist within clonal samples despite elimination of off-types in the source population. Many of these changes appear to be associated with the polyploid genome and compounded by the nature of the adventitiously-based sweetpotato propagation system. We are currently assessing genetic marker uniformity of seed maintained through nodal culture (meristematic tissue origin) versus the conventional method of maintaining sweetpotato seed (adventitious origin). If propagation system plays an important role in sweetpotato clonal variability, then incorporating a method based on preexisting meristematic tissue may reduce inherent genomic variability within foundation seed programs.