Inbreeding depression

But (unlike self-pollinators) cross-pollinators generally do not tolerate much homozygosity. If this is imposed by enforced self-pollination, plant vigour usually declines.

This phenomenon is called "inbreeding depression".

However, when inbred lines are inter-crossed, heterozygosity is restored and plant performance is improved as a result of heterosis.

The molecular basis of heterosis and inbreeding remains controversial.