Complex sentence: All citrus trees are of the single genus "Citrus" and remain almost entirely interfertile; that is, there is only one "superspecies" which includes grapefruits, lemons, limes, oranges, and numerous other types and hybrids.
Split into simpler sentences: All citrus trees belong to the single genus "Citrus" and remain almost entirely interfertile. This means that there is only one superspecies that includes grapefruits, lemons, limes, oranges and various other types and hybrids.