Halkka (1964) determined the formation of one or two chiasmata
in each bivalent in holokinetic chromosome.
and achiasmatic meiosis in African eumastacid grasshoppers.
It is interesting to point out that the presence of two chiasmata
in a variable number of bivalents seems to be frequent in Gagrellinae species, since besides H.
More specifically, we observed two bivalents with two chiasmata
in five diplotene nuclei and one bivalent with three chiasmata
in five diplotene nuclei.
Between the random assortment of homologues, chiasmata
, mutations, and chance combinations of gametes during the process of sexual reproduction, variability is ensured.
Results of chromosome pairing (chromosomal associations and chiasmata
formation per cell during meiosis) and crossability rate (seed set produced by pollinated florets, expressed as percentage) between bread wheat and different Aegilops species, are summarized in Table 4.
Normal meioses in metaphase I contained 24 elements, with 20 circle and 4 chain bivalents and with a chiasmata
number per cell of 44.
And since metaphase pairing is related to pachytene pairing through the formation of chiasmata
, in the frequency of which species and varieties differ genetically, the indication becomes even more uncertain" (Darlington, 1937: 171).
Similarly, recombination is uniform along the chromosome, with a number of chiasmata
drawn from a Poisson distribution with expectation R, where R is the total map length and assuming no interference.
With hindsight, we can now say that the solution was implicit in the same meiotic cells she was studying, because such heteromorphic autosomes, which are very common in grasshoppers (Nur 1961, John 1973, Bidau & Hasson 1984), usually result from heterochromatic supernumerary segments (see below), and the position of chiasmata
(distal or proximal to the segment) determines the mode of segregation of the heteromorphic bivalent during first meiosis (reductional or equational, respectively).
This is the site at which the chiasmata
form, permitting the exchange of DNA between these regions.
In these cells, most of the bivalents possessed an interstitial chiasma with a cross configuration, with the exception of bivalents 1 and 2 that showed two chiasmata
, assuming a ring configuration.
The occurrence of about half rod bivalents is evidence that there may be insufficient chiasmata
to hold quadrivalents together, as is the case in alfalfa (McCoy and Bingham, 1988).
In addition, however, it needs to be stressed that both processes must have profound influences on genetic recombination and segregation by changing the number and nature of linkage groups (interchromosome modification), while the known marked differences in frequency and distribution of chiasmata
in metacentrics as compared with acro- or telocentrics (Mattsson, 1971; Bideau, 1990, 1993) should result in different patterns of intrachromosome recombination.
Diakinetic nuclei were examined for the frequency of chiasmata
(bivalent/trivalent formation) and for frequency and incidence of crossing over within the inverted region of pericentric inversion heterozygotes.