Download Speciation by Changes in Ploidy

WEB TUTORIAL 18.2
Speciation by Changes in Ploidy
Text Sections
Section 18.3 Sympatric Speciation, p. 295
Introduction
New plant species can arise through changes in ploidy—the number of single sets
of chromosomes in a cell or an organism. The new species are reproductively isolated from their parent species, and, once isolated, they evolve independently. This
tutorial illustrates several changes in ploidy that may lead to speciation.
Learning Objectives
•
•
•
Know what is meant by the term "polyploidy."
Understand the consequences of nondisjunction during meiosis.
Discuss how polyploid individuals can arise from diploid individuals.
Narration
Ploidy
Let's use this simplified cell, which represents a diploid plant cell, to examine how
changes in a cell's ploidy can contribute to the formation of new plant species. This
cell has two sets of chromosomes, each set consisting of one red and one blue chromosome. In a pair of homologous chromosomes, one chromosome originates from
the female parent—indicated by the stippling—and the other originates from the
male parent.
When a diploid cell undergoes meiosis, it normally produces four haploid daughter cells, each with one set of chromosomes.
Nondisjunction in Meiosis
Every so often, an error called nondisjunction occurs in meiosis, and some daughter cells get too many chromosomes, while others get too few.
Nondisjunction can occur in meiosis I or meiosis II, but here we'll look at an example in meiosis I. In meiosis I, chromosomes would normally segregate from each
other, with the chromosomes of each homologous pair going to opposite poles.
However, in this case, all the chromosomes migrate to one pole. One daughter cell
has no chromosomes and the other has four.
The cell with four chromosomes continues through meiosis II. The chromosomes
line up at the middle of the cell, and the sister chromatids separate. The cell
divides, producing two cells.
Through nondisjunction, a diploid parent cell has produced two diploid gametes.
In contrast, normal meiosis results in four haploid gametes. They're called gametes
here for simplicity, but keep in mind that in plants the production of gametes
requires many more steps.
What would happen if two diploid gametes came together during fertilization?
Forming an Polyploid Zygote
When the two gametes fuse during fertilization, the resulting zygote is a polyploid
cell, meaning that it has more than two sets of chromosomes. A cell with four sets
is more specifically called a tetraploid.
Meiosis in a Tetraploid
Polyploid plants survive perfectly well, but they cannot produce fertile offspring
when mating with their diploid counterparts. To see why, we'll observe several
rounds of meiosis and fertilization.
First, let's see how a tetraploid undergoes meiosis. Like all cells, it first replicates
its DNA. During meiosis, the homologous chromosomes may come together in
pairs, in fours, or in other arrangements. The cell divides during meiosis I. It
divides again during meiosis II. The result is four diploid gametes.
Fertilization of Diploid Gametes
Let's now consider the act of fertilization. When two normal, haploid gametes fuse,
the result is a normal, diploid zygote.
If two of these diploid gametes came together in fertilization, the result would be
another tetraploid zygote that would continue the generations of tetraploids.
However, when one of these gametes fertilizes a haploid gamete from a normal
diploid plant, the resulting zygote has three sets of chromosomes. It is triploid.
Meiosis in a Triploid
Although a triploid zygote typically develops into a viable plant, meiosis in the
triploid is a confusing affair.
Before meiosis, the cell replicates its DNA. During meiosis I, the homologous
chromosomes try to pair up. Because the extra chromosomes lack homologs; they
may align with the other two, or they may not. The cell divides. Each daughter cell
receives a unique assortment of chromosomes. The cells divide again during meiosis II.
In this case of triploidy, all the gametes have an extra chromosome. In a system
with more chromosomes than this example, every gamete has a different assortment of extra chromosomes. Apparently, this imbalance prevents seeds and pollen
from developing normally. Many so-called seedless cultivars, such as bananas and
watermelon, are triploids.
Tetraploid gametes, on the other hand, have a balanced number of chromosomes.
Because the tetraploid and diploid plants mate to produce sterile triploids, they are
reproductively isolated and are considered two separate species.
Reproductive Isolation of Polyploids
These diploid cells are from two different plant species. If the plants mate and create hybrid offspring, speciation can result. One of the cells has a diploid number of
4 chromosomes, and the other has a diploid number of 6. When these cells undergo meiosis, they produce cells with haploid numbers of 2 and 3 chromosomes,
respectively.
Let's say that pollen from one species fertilizes an ovule from the other. In this
case, the hybrid zygote may be viable, because the sets of chromosomes it receives
from each parent are complete. However, because the chromosomes don't exist in
matching pairs, this zygote is essentially haploid, with a haploid number of 5.
Without pairing chromosomes, a cell cannot divide by meiosis. But it can divide
by mitosis. If a nondisjunction error occurs in mitosis, the cell's ability to divide by
meiosis can be restored.
Before mitosis occurs, the cell replicates its DNA so that each chromosome consists of two chromatids.
The chromosomes move to the middle of the cell. In this case of nondisjunction,
all of the chromosomes move to one pole of the cell as the cell divides.
The daughter cell that receives the chromosomes is now diploid, and it has a
diploid number of 10.
A cell with pairs of chromosomes can divide by meiosis, producing gametes with
5 chromosomes. These gametes can fertilize each other, resulting in a diploid
zygote with 10 chromosomes. This zygote represents the beginning of a new
species. It contains all the chromosomes from its two parent species, but is now
reproductively isolated from its parents because they all have different numbers of
chromosomes.
Species C is an example of a new species that has arisen through polyploidy. It was
formed by combining gametes containing chromosomes of two distinct sets, followed by an error—such as nondisjunction—that resulted in chromosome doubling.
Speciation based on polyploidy is common in wild plant species.
You should now be able to…
•
•
•
•
Diagram the gametes that result from nondisjunction at meiosis I or II.
Explain why a triploid plant is sterile.
Describe how a sterile hybrid plant could give rise to a new species.
Explain why polyploid individuals are reproductively isolated from their
parents.