Heterosis

[GreenMan]

Soooo what exactly causes increased vigor in some F1 hybrids? I am talking about plants.

Is it domianance or overdominance? Even when I read explanations about these two hypothesis,it is not explained in detal,on a genetic and molecular level.Also,what I don't quite understand,when creating vigorous F1 hybrids,do the pure lines *need* to be homozygous to achieve the increased vigor effect,and if so,why? Or is it done to simply impove uniformity?

Any answers and/or sources of latest research on the topic would be greatly appreciated!