Nondisjunction of rye B chromosomes is gametophytically controlled by the transacting control element

Nondisjunction of rye B chromosomes is gametophytically controlled by the transacting control element

Ryota Kousaka and Takashi R. Endo

Laboratory of Plant Genetics, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan

Corresponding author: Takashi R. Endo

E-mail: trendo@kais.kyoto-u.ac.jp

The existence of supernumeral chromosomes, so-called B chromosomes, has been reported in many literatures (Houben et al. 2011; Jones and Puertas 1993; Camacho et al. 2011). In spite of the fact that B chromosomes are dispensable for plant growth and reproduction, they are not only transmitted from generation to generation but also increase in number by nondisjunction in pre- or post-meiotic mitosis. Rye B chromosomes undergo nondisjunction in the first mitosis after meiosis. It has been elucidated that the long-arm distal region (so-called transacting control element) and pericentromeric region (so-called chromatid adhesion site) of the rye B chromosome are critical to nondisjunction. The distal region is characterized by specific repetitive sequences E3900 and D1100 (Sandery et al. 1990). In the first pollen mitosis sister chromatids of rye B chromosomes are attached to each other at the chromatid adhesion site, leading to nondisjunction. In addition, a rye B chromosome with a partial deletion of the long arm increases in number by nondisjunction when it is present together with a complete rye B chromosome in the same plant (Lima-de-faria. 1962). However, it is yet to be identified whether the two regions related to nondisjunction need to be present in one and the same germ-line cells. Male meiocytes are surrounded by tapetum cells that provide proteins and other molecules essential for pollen development (Piffanelli et al. 1998), and therefore it could be possible that the transacting control element present in somatic cells act on the chromatid adhesion sites in meiocytes.

Endo et al. (2008) dissected a rye B chromosome by the gametocidal system in common wheat and produced common wheat lines carrying different segments of the rye B chromosome. Two of these lines had reconstructed chromosomes (B^s-9 and B^s-10) originated from a reciprocal translocation between the rye B chromosome and a wheat chromosome. B^s-9 retains the centromeric region including the chromatid adhesion sites but lacks the distal region including the transacting control element. B^s-10 has the distal region including the transacting control element but lacks the pericentromeric adhesion sites. These lines enabled us to plan a study to address the above-mentioned problem, i.e. whether the nondisjunction of the rye B chromosome is controlled gametophytically or sporophytically. A wheat line disomic for B^s-9 and monosomic for B^s-10 will allow us to create a situation that some of post-meiotic cells receive both B^s-9 and B^s-10 and that others receive only B^s-9. Endo et al. (2008) produced additional wheat lines containing the short arm (B^s-1) and an iso-chromosome of the long arm (B^s-3) of the rye B chromosome. We can use these lines to test whether or not the nondisjunction ability of the rye B chromosome is affected by the ratio of the transacting control element to the pericentromeric adhesion site. The details of the experiments and results are described below.

Plant Materials

We examined the progenies of the following three lines of a common wheat cultivar “Chinese Spring” (abbreviated as CS) in this study: one obtained from the reciprocal crosses between CS and a CS line disomic for B^s-9 and monosomic for B^s-10, one from self-fertilization of a CS line disomic for a normal rye B chromosome (abbreviated as ryeB) and monosomic for B^s-1, and one from self-fertilization of a CS line monosomic for B^s-3. Endo et al. (2008) developed these CS lines carrying the ryeB and its segments.

Karyotype Analysis

We confirmed the karyotypes of the progeny plants by fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH). To detect the ryeB and its segments we used the ryeB-specific tandem repetitive sequence D1100 (Sandery et al, 1990) and total rye genomic DNA as the probes of FISH and GISH, respectively (Fig. 1). The procedures for chromosome preparation and FISH/GISH were according to Sakai et al. (2009).

Transmission of B^s-9 and B^s-10

We examined by FISH/GISH a total of 127 plants of the progeny obtained from the reciprocal crosses between CS and a CS line disomic for B^s-9 and monosomic for B^s-10 (Table 1). Thirty-nine plants had one B^s-9 and no B^s-10. This result showed that B^s-9 did not undergo nondisjunction in the 39 plants. There was only one plant carrying two B^s-9’s, which probably occurred by irregular chromosome pairing of B^s-9 in meiosis, not by nondisjunction. There was one plant carrying four B^s-9’s and one B^s-10 (Fig. 2), and the four B^s-9’s probably originated through two nondisjunction events, that of homologous chromosomes in meiosis and that of sister chromatids in postmeiotic mitosis. These facts suggested that B^s-9 did not undergo nondisjunction when the distal part of the ryeB long arm (not present in B^s-9) was not present in the same postmeiotic cell. Thus, it was suggested that the presence of the transacting control element (B^s-10) in somatic cells was not prerequisite for B^s-9 to undergo nondosjunction. Nine plants had two B^s-9’s and one B^s-10, indicating that B^s-9 underwent nondisjunction when it was present together with B^s-10. On the other hand, when present alone, B^s-9 underwent nondisjunction in only one plant out of 40 plants. Therefore it can be concluded that the nondisjunction of B^s-9 was caused mostly by the transacting control element in the same postmeiotic cells, not in the surrounding somatic cells.

Many plants carried one B^s-9 and one B^s-10 (11 plants) or neither B^s-9 nor B^s-10 (64 plants) (Table 1). Endo et al (2008) reported that almost all the selfed progeny of a CS line disomic for both B^s-9 and B^s-10 had both B^s-9 and B^s-10. The low transmission of B^s-9 in this experiment can be explained by the formation of a trivalent comprising B^s-9, B^s-10 and a wheat chromosome and of a univalent of B^s-9. Such a meiotic configuration would have resulted in many gametes with neither B^s-9 nor B^s-10. Besides, fertile pollens without B^s-9 and B^s-10, i.e. euploid pollen, would have had relatively better chance to take part in fertilization: Clearly B^s-9 and B^s-10 transmitted to the progeny more frequently on the female side than on the male side (see Table 1).

Transmission of ryeB and B^s-1

In the progeny obtained from self-pollination of a CS line disomic for ryeB and monosomic for B^s-1, seven out of 45 plants had four ryeB’s (Table 2). This frequency (16%) was very low compared with that reported for the selfed progeny of a CS line disomic for ryeB (52%, cited from Endo et al. 2008). This result implied that the nondisjunction of ryeB was suppressed by the presence of B^s-1. There were seven plants with three or four B^s-1’s, suggesting that B^s-1 had an adhesion site to undergo nondisjunction on both male and female sides. It may be said that the increase in the number of adhesion sites resulted in the decrease in the nondisjunction frequency of ryeB, i.e. nondisjunction was suppressed by excess adhesion sites.

Transmission of B^s-3

In the selfed progeny (20 plants were examined) of a CS line monosomic for B^s-3, seven plants had one B^s-3 and one plant had the long arm of ryeB. Twelve plants had no additional chromosome. This result suggested that B^s-3 was difficult to undergo nondisjunction in spite of the fact that B^s-3, an isochromosome of the long arm of ryeB, has two of the adhesion sites and two of the transacting control elements. The adhesion site on the long arm might be inactive or much less active than that on the short arm.

B chromosomes have slightly harmful effects on host plants unless they exist in high numbers, and therefore they are expected to be a prospective vector for transgenes (Houben et al. 2011). Present study provided new insights into the nondisjunction mechanism of ryeB in common wheat.

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