The primrose, Primula kewensis, has 36 chromosomes that are similar in appearance to the chromosomes in two related species, P. floribunda (2n=18) and P. verticillata (2n=18). How could P. kewensis arise from these species? How would you describe P. kewensis in genetic terms?

Verified step by step guidance

1. Step 1: Understand the chromosome numbers of the species involved. Both P. floribunda and P. verticillata have 2n = 18 chromosomes, meaning they are diploid species with 18 chromosomes in their somatic cells. P. kewensis has 2n = 36 chromosomes, which is exactly double the chromosome number of the other two species.
2. Step 2: Consider the possibility of hybridization between P. floribunda and P. verticillata. When two different species hybridize, their offspring initially have one set of chromosomes from each parent, resulting in a hybrid with n = 9 + n = 9 = 18 chromosomes, which is typically sterile due to problems in chromosome pairing during meiosis.
3. Step 3: Explore the concept of chromosome doubling (polyploidy). If the sterile hybrid undergoes chromosome doubling, it would have 2n = 36 chromosomes, restoring fertility because each chromosome now has a homologous partner for pairing during meiosis.
4. Step 4: Define P. kewensis genetically as an allopolyploid. This means it contains two complete sets of chromosomes derived from two different species, making it a fertile hybrid with doubled chromosome number.
5. Step 5: Summarize that P. kewensis likely arose through hybridization between P. floribunda and P. verticillata followed by chromosome doubling, resulting in an allopolyploid species with 2n = 36 chromosomes.

Key Concepts

Polyploidy

Polyploidy is the condition of having more than two complete sets of chromosomes. It commonly occurs in plants and can result from the hybridization of two species followed by chromosome doubling. This process can create new species with increased chromosome numbers, such as a tetraploid with four sets of chromosomes.

Hybridization

Hybridization is the crossing of two different species or genetically distinct populations to produce offspring. In plants, hybrids can combine traits from both parents, and if chromosome doubling occurs, the hybrid can become fertile and form a new species, as seen in allopolyploidy.

Allopolyploidy

Allopolyploidy is a type of polyploidy where the chromosome sets come from different species. It arises when two species hybridize and the resulting hybrid undergoes chromosome doubling, restoring fertility. This explains how Primula kewensis (2n=36) could arise from two species each with 2n=18 chromosomes.