Mitotic instability of B chromosomes during embryo development in Locusta migratoria

Abstract

Mitotic instability of B chromosomes during embryo development has been studied in the locust Locusta migratoria. Direct cytological observation of B chromosome nondisjunction in embryos has shown that it occurs in 2.7 per cent of anaphase and telophase cells, and that this frequency is not significantly different among embryos of 5-9 days of development. We have defined three indices which have been shown to be very useful to quantifying mitotic instability: R, the ratio of embryos showing B chromosome instability, M, the median of the distribution of B chromosome numbers in a sample of embryo cells, which has been shown to be a good estimator of the original number of B chromosomes present in the zygote, and MI, the sum of all deviations (in absolute value) of B numbers with respect to M in the same embryo. Mitotic instability of B chromosomes is already apparent in 3-day-old embryos and reaches its maximum value on the fifth day of development. The intensity of mitotic instability, as measured by MI, varies significantly during the developmental period analysed but no definite trend was observed.

Are the dot-like chromosomes in <i>Trinomys iheringi</i> (Rodentia, Echimyidae) B chromosomes?

“…The MI index showed by the dot-like chromosomes in T. iheringi (about 0.09 on average) is of roughly the same order of magnitude as the values reported for mitotically unstable B chromosomes in L. migratoria (Pardo et al, 1995). In the fish Prochilodus lineatus, the MI values were very high in 1979-80 (about 0.5) but very low only 10 years later ( !…”

“…This suggests that assuming that this class resembles the number of dot-like chromosomes originally present in the zygote (or the analyzed tissue) is a parsimonious inference. In mitotically unstable B chromosomes, such as those in the migratory locust Locusta migratoria (see Pardo et al, 1995), this inference seems to be also appropriate. Under this assumption, we can conclude that the dot-like chromosomes in T. iheringi are slightly (5 %) eliminated from the somatic tissues analyzed.…”

“…In each individual, we calculated the mean number of dot-like chromosomes per cell, as well as the median and mode of these distributions. In addition, we calculated the mitotic instability index (MI) proposed by Pardo et al (1995) for quantifying mitotic instability in B chromosomes and, finally, the accumulation index (A), obtained from the difference between mean and median relative to the median, which indicates accumulation if positive or elimination if negative. This last parameter results from assuming that the median number of dot-like chromosomes in the analyzed tissue represents the original number in the zygote.…”

Cytological and developmental analysis of tychoparthenogenesis in Locusta migratoria

“…The B chromosome is also a good marker of the diploidization process because its frequency in diploid cells of haplodiploid embryos is about double that of haploid cells in the same embryos. Bergerard & Seuge (1959) made a similar observation, but they did not consider the mitotic instability of L. migratoria Bs and hence the intraindividual variation in embryos (Viseras et al, 1990;Pardo et al, 1995b). These observations suggest that the most likely cytological mechanism for diploidization is the restitution of the sister products of early cleavage mitosis, as was proposed by Hewitt (1979). This phenomenon, however, does not explain the mena other than restitution might also be involved, the most likely being cell fusion of haploid and dipbid cells to give rise to tripboid cells, although the fusion of haploid cells to become dipboid, or else of diploid cells to form tetraploid cells, cannot be ruled out.…”

Geographic and seasonal variations of the number of B chromosomes and external morphology in <i>Psathyropus tenuipes</i> (Arachnida: Opiliones)

“…Thus, a likely scenario that accounts for all the observations concordantly would be as follows: 1) The number of Bs at the embryonic stage is low (less than two); 2) The number of Bs gradually increases during embryogenesis and earlier developmental period of juveniles by some accumulation mechanism such as mitotic non-disjunction; 3) Increase in numbers ceases after a certain value (6.0 in the Yatsukami population); 4) Females somehow lay eggs with considerably fewer numbers of Bs than they possess. The phenomenon that the number of Bs increases during embryonic and postembryonic development is known in several organisms, such as Crepis capillaris (Rutishauser and Röthlisberger, 1966), Locusta migratoria (Nur, 1969;Kayano, 1971;Viseras et al, 1990;Pardo et al, 1994Pardo et al, , 1995.…”