SLIPPERY ROCK, PENNSYLVANIA
Slippery Rock University
Maize crosses to illustrate epistatic ratios of kernel colors --Shotwell, MA Epistasis, the nonreciprocal interaction between nonallelic genes, is an important concept in genetics, but one that students often struggle to master. Understanding of this concept is greatly aided by laboratory exercises in which examples of epistasis are examined. Kernel color in maize is an ideal phenotype for illustrating epistasis in the teaching lab. Not only are there a variety of independently assorting genes that influence pigment accumulation, but the large number of kernels on each ear allows for the calculation of statistically meaningful phenotype ratios.

One lab in the genetics course I teach is devoted to epistasis, with the exercise centering on kernel color in maize. Over the years, I grew dissatisfied with the quality of the ears I acquired from commercial sources. The colors were often indistinct, and in some cases the phenotype ratios were not as advertised. I therefore decided to do my own crosses to produce ears that illustrated four epistatic interactions: (1) dominant, (2) recessive, (3) duplicate recessive, and (4) dominant and recessive.

In planning these crosses I was assisted by Phil Stinard of the Maize Genetics Cooperation Stock Center. Of the 12 crosses that I completed, only four will be described here. These involved the parental lines listed in Table 1, all of which were acquired from the Maize Genetics Cooperation Stock Center.

The parental lines were crossed in 1999, and the F1 plants were self-fertilized in 2000 to give F2 ears. Owing to the small size of these ears, new F1 crosses were re-done in 2001; these are the F2 ears that are reported here. Table 2 shows the results of the four crosses, and Figure 1 shows representative F2 ears from each cross. In each cross (except for cross 4; see below), there were two genes segregating that affect kernel color.

In cross 4, only white and purple kernels were expected in the F2 generation, but a small number (69 = 1.6%) of red kernels appeared. These red kernels are the result of the segregation of Pr1 alleles. The red kernels were unexpected because parental line 507A(y1) was believed to be Pr1, not pr1. There were thus three genes segregating in cross 4. When the red kernels are combined with the purple kernels, however, the expected F2 phenotype ratio for dominant and recessive epistasis is approximated.

As shown in Table 3, in all four crosses the observed ratio of F2 kernel phenotypes was very close to the expected ratio based on an assumption of recessive epistasis, dominant epistasis, duplicate recessive epistasis, and dominant and recessive epistasis between the two segregating genes (ignoring the third segregating gene in cross 4). These F2 ears are superior to those commercially available, and I have begun using them in my genetics labs. Future crosses will be aimed at getting a better example of dominant and recessive epistasis (the 13:3 ratio). In the meantime, I will make the F1 seeds available to anyone who would like them.

Table 1. Parental lines used in crosses to illustrate epistasis.
 
Stock No. Genotype Phenotype
ACRY A1 A2 C1 R1 Pr1 Y1 purple
506B A1 A2 C1 R1 pr1 y1 red
507A(Y1) A1 a2 C1 R1 Pr1 Y1 yellow
507A(y1) A1 a2 C1 R1 pr1 y1 white
906D A1 A2 CI-1 R1 Pr1 y1 white
910D A1 A2 c1 R1 Pr1 Y1 yellow

Table 2. Phenotypes in the F1 and F2 generations of crosses to illustrate epistasis.
 
Cross no. Parental lines F1 phenotype
F2 phenotypes*
1 906D x 506B white
white
purple
red
     
1809
448
166
2 906D x ACRY yellow
yellow
white
purple
     
1836
574
772
3 507A(Y1) x 910D purple
purple
yellow
 
     
2901
2195
 
4 906D x 507A(y1) white
white
purple
red
     
3530
657
69

*7, 6, 12, and 9 F2 ears were examined for crosses 1, 2, 3, and 4 respectively.

Table 3. Ratio of phenotypes in the F2 generation of crosses to illustrate epistasis.
 
Cross no.
Epistatic interaction
Expected F2 ratio
Observed F2 ratio
1
Dominant
12 white
3 purple
1 red
11.95 white
2.96 purple
1.10 red
2
Recessive
9 yellow
3 white
4 purple
9.23 yellow
2.89 white
3.88 purple
3
Duplicate recessive
9 purple
7 yellow
9.11 purple
6.89 yellow
4
Dominant and
recessive
13 white
3 purple
13.27 white
2.73 purple+red

 
 

Cross 1
906D x 506B
Cross 2
906D x ACRY
Cross 3
507A(Y1) x 910D
Cross 4
906D x 507A(y1)

Figure 1. Representative F2 ears from crosses to illustrate epistasis.
 
 


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