meiosis : is there meiosis without crossing over?
Hans-Cees Speel
reproduction there has to be meiosis. Here chromosome-pairs split
and every half goes to one sex-cell
My first question is: are there species where there is no crossing over in
meiosis? If there are, where can I find references that describe these cases?
My second question is if the chromosomes that are together in a
sex-cell, will be together again in sex-cells the next generation? Is there a
system that takes care of this [wether there is crossing over or not?].
To illustrate the second question a bit:
In sex cell male are 5 chromosomes. In fertilization they meet 5
chromosomes from the female. If the resulting organism produces sex-cells,
will the 5 original male cells be together again? [if there is meiosis or not?]
Does anybody know this?
Hans-Cees Speel
Robert M. Sweeney
As far as I know crossover always occurs. This increases variation and
possible recombinations. This is adaptive. A theoretical species that does
not undergo crossover during meiosis would be less competative as a
species than one that does.
Once the chromosomes are in the cell, there is now way (nor any reason) to
identify which ones were originally maternal and which were paternal. The
paternal and maternal do not group together in subsequent generations.
There is inependent assortment of maternal and patermal chromosomes in
subsequent generations.
Wirt...@aol.com
>I have a question on meiosis.
>My first question is: are there species where there
>is no crossing over in meiosis? If there are, where
>can I find references that describe these cases?
The short answer is yes -- and no. I know of no species where crossing over
is known to be completely absent, but there are many species in which
crossing over is wholly absent in one sex or the other. Indeed, crossing over
is very rarely engaged at equal rates within the two genders in any species.
In distinct contrast to inter-organismal chromosomal recombination (sex),
crossing over is a cell-internal, intranuclear process that appears to be
under active promotion by the processes that regulate meiotic nuclear
division.
If every gene were encoded as an independent chromosome, crossing over would
be unneccessary. But, in mammals, ca. 4000 gene functions reside on a single
chromosome. Otherwise fit alleles potentially suffer the deleterious
consequences of being physically linked with a severe defect resident
anywhere on a chromosome. Crossing over would seem to offer the possibility
of breaking such linkage disequilibria, informationally separating fit
alleles from the accidental consequences of physically residing near a
defect.
The phenomenon of crossing over was discovered during the period 1908-1915 by
Thomas Hunt Morgan and his students. At the time, the discovery that genes
could cross between chromosomes came as a great shock, similar to the
excitement Barbara McClintock's discovery of transposable elements ("jumping
genes") created during the 1970s. Mayr writes in "The Growth of Biological
Thought" (p. 73): "As Muller has characterized it...'the fact that Morgan's
evidence for [genes] crossing over [from one chromosome to the other] and his
suggestion that genes further apart cross over more frequently was a
thunderclap, hardly second to the discovery of Mendelism.'" In the 80 years
since, however, the value and role of crossing over has been steadily
devalued from its earlier presumed importance as a fundamental evolutionary
process.
Crossing over exists, but it is not necessary to the process of meiosis.
Indeed, in Drosophila males, no potential for crossing over exists because no
chiasmata form during meiosis (Dobzhansky, "The Genetics of the Evolutionary
Process", pp. 146-154). (Chiasmata are the visible cytological events where
two arms of homologous chromatids overlap.)
In the males of a great many species, the rates of allelic recombination
(synonymous with an early definition of crossing over) is either much reduced
or altogether absent. In Lepidoptera (butterflies, moths, skippers) however,
the situation is reversed, as was first discovered by Sturtevant in 1915. He
found that female silkworms and wax moths do not undergo allelic
recombination because there is no formation of chiasmata, whereas males do
form chiasmata. In Diptera (flies), males are heterogametic; they possess one
pair of unpaired (non-homologous) sex chromosomes. In contrast, in
Lepidoptera, females are heterogametic. What informational advantage this
sex-linked suppression of chiasma formation confers to the species is not
clear, but the reduction in the formation of chiasmata in the heterogametic
sex is probably not coincidental, but rather "programmed".
Neither is the advantage that crossing over, when it exists, offers the
species clear. Two recent contending hypotheses are those of Maynard Smith
and Bernstein, Hopf, and Michod, each of which is forcefully argued in the
volume, "The Evolution of Sex" (1988, Michod & Levin, eds., Sinauer).
Maynard Smith argues a role and purpose for crossover not unlike sex itself.
In this view, crossing over acts a mutagenesis accelerator by increasing the
rate of allelic recombination, in effect, a kind of sex on top of sex. In
contrast, Bernstein et al., who argue a DNA repair role for crossing over,
also argue that the rates of crossing over are insufficient to maintain the
accelerator of adapation that Maynard Smith proposes. "The fraction of
physical recombination events [the formation of visible chiasmata] that
result in allelic recombination is infinitesimally small...The fraction of
total recombination events that result in [actual] allelic exchange at
meiosis is...less than 2.6 x 10^-6".
These points of observation can be agreed upon however: (i) chiasmata form
during meiosis and, in some species, during mitosis, (ii) the rates of
chiasmata formation are not synonymous with the rates of allelic
recombination, (iii) the process almost certainly has some-but currently
unclear-evolutionary purpose.
The principal argument supporting the last statement is the general
recognition that crossing over is a "promoted" process rather than an
uncorrected, persistent expression of residual error. The evidence for the
active intranuclear promotion of crossing over is two-fold: (i) proteins with
strand exchange activity have now been identified in organisms as diverse as
the bacteriophage T4 (a virus), bacteria, lower eukaryotes, and human cells.
This commonality implies that DNA strand exchange is a property of a
ubiquitous class of proteins that can be referred to as recombinases. And,
(ii) the crossing-over process must be presumed to be under active promotion
because it can be so easily suppressed in one sex or the other. Only an
actively coded process can be easily suppressed. A random error process
cannot likely be subject to easy sex-specific suppression. If it were, and if
the process had no positive evolutionary value, it would quickly disappear in
both sexes.
It is however also possible to argue with some much lesser force of logic
that current evidence suggests no evolutionary role for crossing over. It is
simply a cytological process that results in no significant genomic
alterations. In human females, the rate of chiasma formation is 1.89 per
bivalent. If the rate of allelic recombination is as low as Bernstein et al.
calculate, the process is insignificant in its effects when compared to
sexually-mediated chromosomal recombination. DNA cannot be broken at any
arbitrary point. The mechanism that allows true allelic recombination to
occur at all is perhaps no more complicated than the majority of the DNA on a
chromosome is pseudogenetic and no longer actively translated. Breaking such
structure is quite likely to be often informationally neutral, and thus
tolerable.
Among the more intriguing aspects of crossing over is that it is almost
always differentially expressed in the two sexes, and in that, shares broad
characteristics with a number of other genetical phenomena. Haplodiploidy is
a sex-differentiated phenomenon that differentially exposes defects in the
male genome to direct selection. Under haplodiploidy, males are haploid and
thus practical allelic recombination during meiosis is functionally
impossible. Parahaploidy is a directly related condition that has been found
to exist in some mites. In this condition, male and female zygotes are fully
diploid, but the male sheds his paternal genome sometime prior to sexual
maturation, and thus allelic recombination in the male is again impossible.
In all placental mammals, the rate of chiasma formation during meiosis is ca.
30% higher in the female than the male. Achiasmatic meiosis is the rule in
most insects which are male heterogametic. The sum of these phenomena suggest
that crossing over serves an informational maintenance purpose rather than
act as the accelerator of adaptation that Maynard Smith has suggested.
>My second question is if the chromosomes that are
>together in a sex-cell, will they be together again in
>the sex-cells the next generation?
Beginning from any one pre-meiotic, fully diploid cell, 2^k gametes may be
produced, where k is the haploid number of chromosomes in the cell. If the
segregation of the homologous chromosomes is completely random (please see
note below), then there a 1 in 2^k chance of creating a gametic cell with
precisely the same complement as fathered (or mothered) you (depending on
your sex).
If you were male, human, and assuming you produce a standard 200 - 500
million spermatozoa per ejaculate wad, you will produce ca. 30-60 gametes in
each ejaculate that are identically complemented to the spermatozoon that
fathered you.
However, these 60 gametes face long odds of actually being the fertilizing
spermatozoon. Only ca. 2000 spermatozoa, out of the 200 million, reach the
site of the ovum and only one, at most, will fertilize it.
>Is there a system that takes care of this
>[whether there is crossing over or not?].
Only luck. Furthermore, the above calculation presumes no crossing over
events. If crossing over had occurred, the identical reconstruction of your
father's gamete is virtually impossible.
Wirt Atmar
A small note on segregation randomness: in general, I believe that randomness
in homologous chromosmal segregation can easily be presumed in most cases,
but there do exist notable exceptions that are now well documented, one of
which is parahaploidy, as discussed above. Another condition is
hybridogenesis, which is a condition found in some all-female unisexual fish,
such as the Amazon molly. Under hybridogenesis, the paternal genome is
non-randomly segregated and is lost during meiosis, generation after
generation. The only chromosomes that are inherited are maternal by descent.
In every case where non-random segregation of one parent's genome or the
other is known to occur, it is the paternal genome that is lost.
ROBERT SAUNDERS
>From: ha...@sepa.tudelft.nl (Hans-Cees Speel)
>Subject: meiosis : is there meiosis without crossing over?
>Date: 27 Sep 1995 21:07:09 GMT
>I have a question on meiosis. Consider a sexual reproducing species. For the
>reproduction there has to be meiosis. Here chromosome-pairs split
>and every half goes to one sex-cell
>My first question is: are there species where there is no crossing over in
>meiosis? If there are, where can I find references that describe these cases?
There is no crossing over in meiosis in Drosophila melanogaster males.
Robert
Keith Robison
: My first question is: are there species where there is no crossing over in
There is little-to-no recombination during meiosis in Drosophila _males_.
: My second question is if the chromosomes that are together in a
: sex-cell, will be together again in sex-cells the next generation? Is there a
: system that takes care of this [wether there is crossing over or not?].
: To illustrate the second question a bit:
: In sex cell male are 5 chromosomes. In fertilization they meet 5
: chromosomes from the female. If the resulting organism produces sex-cells,
: will the 5 original male cells be together again? [if there is meiosis or not?]
Any such segregation bias would show up quickly in a cross (i.e.
linkage between markers on different chromosomes) -- I don't
believe there is anything like that observed.
Keith Robison
Harvard University
Department of Cellular and Developmental Biology
Department of Genetics / HHMI