Hybrid Speciation - Genetics of Hybridization

Genetics of Hybridization

Many agricultural crops are hybrids with double or even triple chromosome sets. Having multiple sets of chromosomes is called polyploidy (or polyploidity). Polyploidy is usually fatal in animals where extra chromosome sets upset fetal development, but is often found in plants. A form of hybrid speciation that is relatively common in plants, occurs when an infertile hybrid becomes fertile after doubling of the chromosome number.

Hybridization without change in chromosome number is called homoploid hybrid speciation. This is the situation found in most animal hybrids. For a hybrid to be viable, the chromosomes of the two organisms will have to be very similar, i.e., the parent species must be closely related, or the difference in chromosome arrangement will make mitosis problematic. With polyploid hybridization, this constraint is less acute.

Super-numerary chromosome numbers can be unstable, which can lead to instability in the genetics of the hybrid. The European edible frog appears to be a species, but is actually triploid semi-permanent hybrids between pool frogs and marsh frogs. In most populations, the edible frog population is dependent on the presence of at least one of the parents species to be maintained as each individual need two gene sets from one parent species and one from the other. Also, the male sex determination gene in the hybrids is only found in the genome of the pool frog, further undermining stability. Such instability can also lead to rapid reduction of chromosome numbers, creating reproductive barriers and thus allowing speciation.