Andrew Smith, of Karori, asks :-

Why is the number of segments in a grapefruit variable?

Annette Richardson, a plant breeder at Plant and Food Research, Keri Keri, responded.

One might expect it to be fixed genetically like the number of fingers on a hand. From observation it can commonly vary between 10 and 13 even for the same variety (eg Red Ruby).

In many plants, for example grapefruit or kiwifruit, the size of the fruit is determined during flower ovary development. While there is a genetic blueprint for what the flower/fruit will look like there can be some variation in the plan during flower development.

Things that cause such variation are carbohydrate supply, hormones and environmental conditions like temperature and water. The number of segments in a grapefruit, derived from the number of carpels in the flower, is not critical for the success of the plant and therefore can vary. The fact that there are segments and that they carry and develop seeds for reproduction is important in an evolutionary sense. This contrasts with, for example, the number of fingers on a person which is probably an important evolutionary trait.