M. C. Pardo

Population Dynamics of A Selfish B Chromosome Neutralized by the Standard Genome in the Grasshopper Eyprepocnemis Plorans

Effects of the B chromosome polymorphism of the grasshopper Eyprepocnemis plorans were analyzed in two natural populations. Postmating sexual selection, female fertility, and survival were studied. The B chromosome lacks drive and has no detectable effects on fitness. A neutral B cannot invade a population and establish a polymorphism, but the confidence limits on our estimates cannot exclude the possibility that the polymorphism is maintained by a balance between weak drive and weak selection against individuals with two and three Bs. However, other lines of evidence favor the following model of the dynamics of the B in E. plorans. In a newly invaded population, the B has substantial drive, but the evolution of drive suppressor genes in the A chromosomes neutralizes the B drive so that it becomes near‐neutral and begins a random walk toward extinction by stochastic loss. Because the B is common by the time drive disappears, the random walk is likely to continue for a long time. If in the course of the random walk a variant B with greater drive appears, then it will displace the original variant, and a new cycle of drive suppression and drift to extinction occurs. A simulation model of this process suggested that the mean time to extinction is proportional to the two‐thirds power of the population size; it is much less affected by subpopulation size or the number of populations in a subdivided population.

Generating high variability of B chromosomes in Eyprepocnemis plorans (grasshopper)

Twenty-eight progeny analyses (PAs) performed on specimens of E. plorans collected from four natural Iberian populations have been informative about the transmission of rare B chromosome types or the de novo origin of some of them. At least 11 rare B-types have been found in addition to the predominant ones: B1 in Daimuz, B2 in Jete and Salobreña, and B5 in Fuengirola. The presence in two controlled crosses of one embryo carrying a B-type which was absent in the parents suggests that these B variants (B2iso and B1f1) have originated de novo. Eleven other PAs suggest that new B derivatives are recurrently arising in these populations. The most frequent B chromosome mutation was centromere misdivision that originated four different B-types (B2m1, B1iso, B2iso and Bmini). Other rearrangements were pericentric inversions (B2i1, B2i2 and B2i3), inverse tandem fusion (B2it1), centric fusion (B1f1) and deletions (B2d1 and B2d2). The four B derivatives produced by centromeric misdivision are significantly eliminated during sexual transmission, most probably owing to deficiencies in the control of chromosome movement by their hemicentromeres. Those derived from translocations showed Mendelian transmission but deletion B variants showed a tendency to elimination. Our results suggest that B chromosome substitution of B1 by B2 in the Salobreña and Jete populations could be achieved by differences in relative transmission efficiency, as in one controlled cross, where the female carried 1 B1 plus 1 B2, B2 was significantly overtransmitted and B1, eliminated.

Female fitness is increased by frequent mating in grasshoppers

Radiolabelling experiments with Eyprepocnemis plorans demonstrated that males transfer protein-aceous substances with the ejaculate that are subsequently incorporated into the eggs that the females lay. The amount of male radioactive nutrients detected in the eggs was positively correlated to mating frequency, and the proportion incorporated was almost twice that of egg weight relative to female weight. This might suggest the existence of a specific mechanism selectively incorporating male ejaculate nutrients into the eggs. Another series of experiments with four types of female differing in mating frequency and male availability demonstrated that female production of eggs and embryos is directly proportional to mating frequency. This improvement in female fecundity is from the ejaculate nutrient transfer and both effects are directly dependent on mating frequency. Furthermore, in females crossed sequentially with two different males, there was a significant increase in mating frequency when the female was given the second male. This could be due to an increased mating effort of the second male to displace paternity of the previous male. The low paternity confidence and the polygynandric mating system that characterize E. plorans suggest that male ejaculate nutrients most likely function as a kind of mating effort rather than paternal investment.

Paternity displacement in the grasshopper Eyprepocnemis plorans

Three types of double crosses were carried out to investigate sperm competition in the grasshopper Eyprepocnemis plorans. Maximum likelihood estimation of paternity probability showed a high degree of second male sperm precedence (P2 = 90%). The results also showed that: (i) a single copulation may be enough for an efficient paternity displacement; (ii) males do not use sperm plugs, but they mate for a prolonged period of time to resist further copulation; and (iii) successive paternity displacements may be seen in the same female. The mechanisms controlling sperm competition in E. plorans, and the evolutionary implications of this phenomenon are discussed

Mitotic instability of B chromosomes during embryo development in Locusta migratoria

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.

Random mating and absence of sexual selection for B chromosomes in two natural populations of the grasshopper Eyprepocnemis plorans

The effects of B chromosomes on mating behaviour in natural populations of the grasshopper Eyprepocnemis plorans were investigated by collecting mating pairs in the field. The results have shown random mating and the absence of sexual selection related to the presence of Bs. Possible hypotheses explaining the maintenance of this polymorphism, despite the absence of drive and sexual selection, are discussed.

Cytological and developmental analysis of tychoparthenogenesis in Locusta migratoria

A number of females of Locusta migratoria have shown tychoparthenogenetic reproduction (a kind of accidental thelytoky) characterized by: (i) a female-biased primary sex ratio; (ii) the production of embryos with abnormal ploidy levels, mainly haplodiploid mosaics; (iii) a longer time for embryo development; and (iv) the capability of producing female offspring that reproduced in the absence of males. Perfect diploidization is not essential for parthenogenetic embryos to reach the adult stage but a great majority of embryo cells must be diploid to complete properly embryogenesis and hatch. In addition, diploidy is apparently necessary to the ovary of parthenogenetic females so that eggs laid without mating can hatch. Cytological analyses of embryos at different developmental ages have shown that parthenogenetic embryos begin haploid and gradually become diploid, thus passing through a haplodiploid mosaic stage. The most likely mechanism for ploidy restoration is the restitution of the sister products of cleavage mitoses, although our results show that cell fusion could be another mechanism, the relative importance of which remains to be tested in future work. Although parthenogenetic females showed a fecundity comparable to that of sexual females, their reproductive output was significantly lower because of a decrease in the number of embryos per pod and a consequent decrease in the rate of embryo production.

Dynamics of sperm storage in the grasshopper Eyprepocnemis plorans

Abstract. Sperm storage duration in the grasshopper Eyprepocnemis plorans Charpentier (Orthoptera, Acrididae) was found to be 58.5 days but with a wide between‐female variation (from 26 to 113 days). One copulation was enough to fertilize two to eight egg‐pods and to produce 130 eggs (28–313) and 119 embryos (28–290), on average. Sperm availability, however, decreased progressively with time, so that the majority of females spent a long final period of their lives without laying any pod. Finally, sperm storage duration was positively correlated with clutch size and total production of eggs and embryos.

Fitness effect analysis of a heterochromatic supernumerary segment in the grasshopper Eyprepocnemis plorans

Several components of fitness were analysed in relation to the presence of a supernumerary chromosome segment (SCS) in two natural populations of the grasshopper Eyprepocnemis plorans, including clutch size, egg fertility, egg and embryo productivity and survivability from embryo to adult, and SCS transmission through males. The results have shown the absence of a significant relationship between SCS presence and these fitness components, with the single exception of egg fertility which decreases significantly in SCS females with mating shortage. This fertility decrease is thus expected to be relevant for the population dynamics of the SCS only in low-density populations, those in which it is difficult for females to find a male to copulate with before each egg-batch is ready to be laid. The analysis of the SCS transmission through males showed no significant differences between expected and observed SCS frequencies. The SCS polymorphism seems to be at a status close to neutrality in respect to fitness, but its slight disadvantage in transmission through females carrying B chromosomes predicts that the polymorphism should tend to disappear, unless SCS recurrent amplification, or another undiscovered force, counteracts this tendency.