It's the '+' operator at the beginning of the line that trips things up (not just that you are using two '+' operators consecutively). The '+' operator can be used at the end of lines, but not at the beginning.
This works:
ggplot(combined.data, aes(x = region, y = expression, fill = species)) +
geom_boxplot()
The does not:
ggplot(combined.data, aes(x = region, y = expression, fill = species))
+ geom_boxplot()
*Error in + geom_boxplot():
invalid argument to unary operator*
You also can't use two '+' operators, which in this case you've done. But to fix this, you'll have to selectively remove those at the beginning of lines.