To determine whether the mutations in the seven strains leading to the wingless phenotype are in the same gene, your lab sets up complementation tests. You obtain the following results ("+" indicates that complementation was observed; "-" indicates that no complementation was observed).

mutation 1 2 3 4 5 6 7
1 -
2 - -
3 + + -
4 - - + -
5 + + - + -
6 + + - + - -
7 - - + - + + -
How many different genes are contributing to the wingless phenotypes in these mutant fly strains?