Preheat oven to 425 degrees F.
Grease a 12 muffin (2 1/2") tin.
Combine mix and sweetener equivalent to ¼ cup sugar in bowl; add yogurt.
Add egg; beat with fork until well combined.
Fold berries gently into batter with rubber spatula; place about ¼ cup batter into each muffin cup.
Combine lemon peel and remaining sweetener and mix well.
Sprinkle over batter; bake until golden brown (20 to 25 minutes).
Yields 12 muffins.