Biology Reference
In-Depth Information
3.16 B Chromosomes
B chromosomes
are a heterogeneous class of chromosomes found in the nucleus
and also are called accessory or supernumerary chromosomes. B chromosomes
are found in many insects, and they probably originate by several mechanisms,
including being derived from autosomes and sex chromosomes in intra- and
interspecies crosses (
Camacho et al. 2000
).
B chromosomes may only be present in some individuals from some popula-
tions in a species. B chromosomes have irregular mitotic and meiotic behavior,
thereby allowing them to accumulate in the germ line, so that they are trans-
mitted at rates higher than those of normal chromosomes.
Over evolutionary time, genes on B chromosomes may be silenced, undergo
heterochromatinization, and accumulate repetitive DNA and transposons.
B-chromosome frequencies in populations result from a balance between their
transmission rates and their effects on host fitness. The long-term survival of B
chromosomes depends on their ability to survive efforts by their host to eliminate
or suppress them because they are often considered to be parasites. Because B
chromosomes can interact with standard chromosomes, they could play a positive
role in genome evolution if they contribute useful genetic information.
An example of a very interesting B chromosome is that of the paternal sex-
ratio (PSR) chromosome of the parasitoid
Nasonia
. PSR increases in frequency
because it is able to destroy paternal chromosomes (described in Chapter 10).
The effects of other B chromosomes on their hosts are often unknown.
3.17 Sex Chromosomes
In eukaryotes with identifiable sexes, there generally is a pair of chromosomes
called
sex chromosomes
. The sex chromosomes are often morphologically dif-
ferent from the rest of the chromosomes (
autosomes
). In most species, the male
is the
heterogametic sex
, meaning the sex that has heteromorphic, only par-
tially homologous sex chromosomes that are usually called X and Y. The X and
Y chromosomes pair in the first prophase of spermatogenesis and, as a result
of segregation, two types of gametes are produced: one type containing the
X chromosome and one type containing the Y chromosome. Sperm containing
the Y chromosome will result in male progeny, whereas sperm containing the
X chromosome will produce female progeny. Some heterogametic species have
males that are XO, lacking a Y chromosome.
Typically, the Y chromosome is smaller than the X chromosome and has very
few of the genes that are on the X chromosome. The Y chromosome is often
