Plant hybrids are the result of sexual reproduction between plants from two different taxa or species. Not all plant hybrids are sterile, but many are. Sterility in plant hybrids is most often the result of polyploidy, which occurs because of abnormal cell division and results in more than two sets of chromosomes in the cells of the hybrid offspring. Hybrids commonly form in nature between closely related species, but humans also produce sterile hybrid plants intentionally for commercial purposes.
Most organisms are diploid, which means they have two complete sets of chromosomes. When plants have extra sets of chromosomes, they are polyploid. Polyploidy is the result of accidents during cell division. Cells must divide in order to make gametes (egg and sperm cells) and when accidents occur, extra sets of chromosomes can create a polyploid state in the hybrid offspring.
Hybrid plants are sterile when they have the incorrect number of chromosomes (which results from polyploidy). If a plant has uneven numbers of chromosome pairs, it can’t produced balanced gametes (egg or sperm cells) and will not be able to produce viable offspring.
Hybridization in plants is not necessarily detrimental. In fact, humans selectively breed many plants with the purpose of creating hybrid offspring because hybridization can sometimes improve seed or fruit production or resistance to disease. Hybrids also often exhibit larger and flashier flowers than the parent species. Sterile hybrid fruit crops are intentionally produced in order to produce fruits without seeds for commercial purposes.
Not all plant hybrids are sterile. Polyploid hybrids commonly form in nature in plants, and hybrid plants can be fertile when they are crossed with other polyploid plants with even numbers of chromosomes. In order for plants to be fertile, they must be able to produce gametes with balanced numbers of chromosomes in their cells.
According to the 2009 book “Why Evolution is True,” hybridization is an important mechanism in evolutionary diversity in plants. Polyploidy in plants is an important mechanism for sympatric speciation, or the branching of new species from ancestral ones within the same geographic area. Polyploidy results in reproductive isolation from the parent population, which means that the polyploid plants can’t mate with the parent plants. Over time, reproductive isolation, combined with natural rates of genetic mutation that occur in all organisms, can result in the appearance of a new species of plant that is genetically distinct from the ancestral species.