The chiasmatype theory. A new interpretation of the maturation division.
F.A. Janssens (1909)
Cellule 25: 387-411
(A few selected passages freely translated from the French by
Romain Koszul, Karine Van Doninck & Matthew Meselson plus explanatory notes in CAPS)
Following the theoretical considerations of Weismann, many authors agreed that the
hereditary forces must be embodied in the germinal plasma in the form of material particles
present in the sexual elements that contribute to the formation of the fertilized egg. It is in the
chromatic part of the nucleus that these representative particles should be looked for. Recent
studies on maturation divisions have strongly expanded these ideas and proved the outstanding
creativity of Weismann’s theory.
The strongest support for this theory is the wonderful concordance that exists between the
results of Mendel’s studies, obtained at a time where cytological studies had only just begun, and
the results subsequently obtained from such studies even before the rediscovery of Mendel’s
work and laws.
This concordance has been clearly stated by Boveri. He indeed asserts that Mendelian
allelomorphic characters are carried by comparable chromosomes in the nucleus.
It is known that in many plants and animals the shapes and lengths of chromosomes vary
greatly. Many recent works support the following statements:
1. A chromosome of specific shape and length has a homolog (twin) in every somatic cell
(except for accessory chromosomes).
2. Homologue (twin) chromosomes have different origins. One is provided by the egg and
therefore comes from the mother, whereas the other was brought by the spermatozoan that
fertilized the egg and is therefore from the father.
3. In an early step in the maturation of oocytes and spermatocytes conjugation of similar
chromosomes takes place.
4. At a later stage, pachytene, those chromosomes begin to separate but are still linked
together and more or less coiled.
5. During this first division there occurs the qualitative reduction postulated by Weismann
and already foreseen by Mendel in his law of character segregation in gametes.
6. Therefore the resulting secondary spermatocytes and oocytes are of pure lineage (Mendel
states that gametes are always of pure lineage), and the division that follows will simply separate
the chromatids. This is a division with the same longitudinal cleavage of chromosomes found in
any division. It is therefore equational (Roux)...
However, this elegant and simple theory has never fully satisfied us, at least regarding the
two maturation divisons. Therefore, we investigated in this new, in-depth study, the hetero- and
homeotype division. In this short note we express our concerns about the current theory and
discuss the main results of our research. [HETERO- AND HOMEOTYPE ARE OBSOLETE
TERMS FOR THE FIRST AND SECOND DIVISIONS OF MEIOSIS, RESPECTIVELY.]
...
[JANSSENS THEN GIVES THE FOLLOWING THREE THEORETICAL REASONS FOR
QUESTIONING THE PREVAILING DESCRIPTION OF MEIOSIS, LEADING TO HIS IDEA
THAT CHROMATIDS BREAK AND JOIN IN THE FIRST MEIOTIC DIVISION, CAUSING
WHAT IS NOW CALLED CROSSING-OVER.]
The tetraspore is found in all plants and animals and also very likely in the most evolved red
algae. This is therefore of capital importance. If four and not two spores are formed, then each of
them must have something unique. However, all the modern cytological studies have concluded
that there are only two types of spores among the four...
...
The significance of the typical coiling or strepsitene step [ROUGHLY, PACHYTENE
AND DIPLOTENE] is not understood. If the only achievement of the whole process is to
separate two chromosomes that have , it looks like a minor result for weeks or sometimes monthlong efforts...
Finally, present theory provides an explanation of Mendel’s law that is definitely
interesting but is incomplete. Indeed, cases have been reported in which there are more
allelomorphic characters that segregate independently than there are pairs of chromosomes.
...
[AT THIS POINT JANSSENS PRESENTS WHAT HE BELIEVED TO BE CYTOLOGICAL
EVIDENCE FOR HIS INTERPRETATION OF CHIASMATA. THESE ARE PLACES
WHERE TWO OF THE FOUR CHROMATIDS APPEAR TO BE STUCK TOGETHER
DURING THE LONGITUDINAL SEPARATION OF CHROMOSOMES LATE IN THE
FIRST MEIOTIC DIVISION. JANSSENS BELIEVED THAT AT SUCH SITES ONE
PATERNAL AND ONE MATERNAL CHROMATID BREAK AND THEN REJOIN CROSSWISE -- MATERNAL TO PATERNAL -- GIVING WHAT GENETICISTS LATER CALLED A
"CROSS-OVER". ALTHOUGH HIS CYTOLOGICAL EVIDENCE WAS MERELY
SUGGESTIVE, IT IS CORRECT THAT CHIASMATA CORRESPOND TO SITES OF
CROSSING-OVER BETWEEN CHROMATIDS.]
"...it is extremely difficult to say which of the two chromosomes located at a chiasma is
located above or under the other one. Anyway, chromosomes are more or less interlaced
with each other at these locations.
...
We believe that in this case the filaments which intersect, are those further apart, i.e.
which occupy those parts of the chromosomes that undergo no intermixture. The filaments which
remain unconnected by a chiasma, on the contrary, are those that have undergone a secondary
union at the points where the chromosomes have interpenetrated and fused. The schemas XIII,
XIV and XV represent, at the point of chiasma, the gradual intermixture of two chromosomes,
with union of the first two filaments that touch each others.
...
Therefore, the full chromosome will split into two segments that will fuse to the ones of its
neighbor and generate new combinations of chromosomal segments. For example, let’s consider
that a chromosome, composed of segments A and B, is associated in a dyad with a chromosome
composed of segments a and b. After splitting and secondary fusions, the dyad will be composed
of chromosomes Ab and aB.
...
A. As exposed in this brief statement, the relationships between chromosomes in dyads are
far from being as simple as commonly believed until now. When chromosomes touch each other
at chiasma, which is, according to us the rule, we do not believe they remain independent. Their
filaments are subject to contacts that can modify their connections from one segment to the next
one. This will result into new segmental combinations...
***