Plant species regularly hybridize in nature, but only some crosses result in new polyploid species. Of the crosses below, if each one occurred, which would be most likely to result in a new stable polyploid species?

Question

A.  A cross between a 2n gamete and a 1n gamete, forming a 3n hybrid.
B.  A cross between a 4n gamete and a 1n gamete, forming a 5n hybrid.
C.  A cross between a species with a CC genome and a species with a DD genome, forming a hybrid with a CD genome.
D.  A cross between a species with a BB genome and a species with a DD genome, followed by a doubling of chromosomes, forming a hybrid with a BBDD genome.

Clarify Question
  · What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain and what do they mean?
 
Gather Content
· What do you already know about polyploidy?
 
Consider Possibilities · Consider the different answer options. Which can you rule out?
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

D.  A cross between a species with a BB genome and a species with a DD genome, followed by a doubling of chromosomes, forming a hybrid with a BBDD genome.

Clarify Question
  · What is the key concept addressed by the question?
        o This question addresses polyploidy.
· What type of thinking is required?
        o This question is asking you to weigh and judge, or evaluate, evidence to choose the best of the possible answers.
· What key words does the question contain and what do they mean?
        o Plant species hybridize in nature – that is, two different species can mate to produce hybrid progeny.
        o A polyploid species is a species that is stable and can reproduce, despite having a greater-than-diploid (2n) number of chromosomes.
 
Gather Content
· What do you already know about polyploidy?
        o Polyploidy has occurred numerous times in the evolution of the flowering plants.
        o Allopolyploids arise by hybridization of two lineages followed by genome duplication.
        o Unless the hybrid genome is doubled, a hybrid plant will be sterile because it will lack homologous chromosomes that need to pair during metaphase I of meiosis.
        o Because meiosis requires an even number of chromosome sets, only species with ploidy levels that are multiples of two can reproduce sexually.
 
Consider Possibilities · Consider the different answer options. Which can you rule out?
        o Could a 3n hybrid form a new polyploid species? Unlikely, because at meiosis the 3 sets of chromosomes will not assort evenly, and gametes will be aneuploid.
        o Could a 5n hybrid form a new polyploid species? Unlikely, because at meiosis the 3 sets of chromosomes will not assort evenly, and gametes will be aneuploid.
        o Could a CD hybrid form a new polyploid species? Unlikely, because at meiosis the two different sets of chromosomes (denoted by “C” and “D”) will not assort evenly, and gametes will be aneuploid.
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Could a BBDD hybrid form a new polyploid species? Yes, because at meiosis the B chromosomes can pair together, and the D chromosomes can pair together, forming euploid BD gametes.
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked which hybrid would be most likely to become a new stable polyploid species.
        o The question required you to weigh and judge, or evaluate, evidence to choose the best of the possible answers.
        o Did you recognize that a polyploid has to have a ploidy that is an even number, to have matched chromosomes for meiosis?