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

Abstract. Polymorphism analysis and yield tests were conducted among 'Jewel' sweetpotato clones [Ipomoea batatas (L.) Lam] obtained from eight state foundation seed programs. Initially, 38 arbitrary primers generated a total of 110 scorable DNA fragments in a sample of virus-indexed plants from each clone source. The number of marker loci scored for each primer varied from one to eight with an average of 2.89. Twenty-one bands (19.1%) were scored as putative polymorphic markers based on the presence or absence of amplified products. Further estimation of variability within each clone source was accomplished by an assay of 10 sample plants per clone group by 14 marker loci generated by four selected primers. Polymorphic bands ranged from 7.1% to 35.7% in five of eight clone groups. Field studies show variation in nearly all yield grades measured. In three tests during the 1991 and 1992 seasons, yield differences ranged from 27% to 46% within the economically important U.S. no. 1 root grade. The results suggest the usefulness of arbitrarily-primed markers in detecting intra-clonal sweetpotato DNA polymorphisms and indicate an underlying genetic cause for phenotypic variability in the crop.

Asexual propagation theoretically preserves genotypic identity and uniformity within a clonal cultivar. In the sweetpotato, the zygotic stage determines a cultivar's genotypic constitution; ensuing mitotic duplications perpetuate this unique genetic makeup. Adventitious sprouts derived from fleshy roots of sweetpotato are used for vegetative propagation, hence conservation of cultivar genetic identity is expected. However, phenotypic plasticity especially in quantitatively inherited traits is expected due to environmental effects. Several reports document the magnitude of environmental influence, expressed as genotype x environment interactions, in sweetpotato phenotypic expression (Collins et al., 1987; Huett, 1976; Kannua and Floyd, 1988; Ngeve and Bouwkamp, 1993). Nevertheless, the yield of a well-established sweetpotato cultivar can range from zero to several kilograms per plant. Slight differences in size and quality among sprouts or cuttings used in plantings cannot account for all of this observed variability (Hwang et al., 1983; Steinbauer et al., 1943). More importantly, visible qualitative variability such as root, skin, and flesh color changes are common. For instance, flesh color mutation rates in sweetpotato range between 1% and 18%, depending on the cultivar (Hernandez et al., 1964). In contrast, the estimated mutation rate in clonally-propagated potato (Solanum tuberosum) is one in 100,000 to 200,000 plants (Heiken, 1958). Consequently, sweetpotato foundation seed programs allocate substantial resources in maintaining the genetic uniformity of cultivars.


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: 1) 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.

Literature Cited