B chromosome in Plantago lagopus Linnaeus, 1753 shows preferential transmission and accumulation through unusual processes

Abstract

Plantago lagopus is a diploid (2n = 2x =12) weed belonging to family Plantaginaceae. We reported a novel B chromosome in this species composed of 5S and 45S ribosomal DNA and other repetitive elements. In the present work, presence of B chromosome(s) was confirmed through FISH on root tip and pollen mother cells. Several experiments were done to determine the transmission of B chromosome through male and female sex tracks. Progenies derived from the reciprocal crosses between plants with (1B) and without (0B) B chromosomes were studied. The frequency of B chromosome bearing plants was significantly higher than expected, in the progeny of 1B female × 0B male. Thus, the B chromosome seems to have preferential transmission through the female sex track, which may be due to meiotic drive. One of the most intriguing aspects of the present study was the recovery of plants having more chromosomes than the standard complement of 12 chromosomes. Such plants were isolated from the progenies of B chromosome carrying plants. The origin of these plants can be explained on the basis of a two step process; formation of unreduced gametes in 1B plants and fusion of unreduced gametes with the normal gametes or other unreduced gametes. Several molecular techniques were used which unequivocally confirmed similar genetic constitution of 1B (parent) and plants with higher number of chromosomes.

Origin, Behaviour, and Transmission of B Chromosome with Special Reference to Plantago lagopus

“…Drive leads to an increased number of Bs in next generation and the maximal number of Bs that can be tolerated by the host varies between species e.g., maize has 34 while rye has six B chromosomes. In this regard, P. lagopus is unique in having 2 B chromosomes at diploid and 5 B chromosomes at triploid levels [ 73 ]. It depends on a balance between B chromosome accumulation due to drive, and negative effects on fertility and vigor that are caused by the B chromosome [ 5 ].…”

“…The mechanism of accumulation of the B chromosome in P. lagopus was recently reported in depth by Dhar et al, (2017) by performing extensive crossing experiments and calculating the transmission of B chromosome through female and male sex tracks [ 73 ]. This study not only explored the existence of drive in B chromosome of P. lagopus , but also helped in understanding the mechanism of perpetuation in the populations.…”

“…It was reported that in P. lagopus when 1B plant was used as a male, transmission rate was in accordance with the expected Mendelian ratio. The differences in segregation ratio observed among various cross combinations were attributed to the heterozygous nature of P. lagopus —being a cross-pollinated species [ 73 ]. Alternatively, when 1B plants were used as female, there were significant deviations from the 1:1 ratio; frequency of B chromosome bearing plants was higher than the Mendelian expected value.…”

“…The fact that during female meiosis in plants, only one out of the four meiotic products survives can explain this drive through the female sex tracks. The B chromosome of P. lagopus is perpetuated preferentially due to drive, expressed during both male and female meiosis [ 73 ]. The same phenomenon has been observed in rye where the drive happens in both male and female sex tracks [ 84 ].…”

“…In P. lagopus , the entire complement of the species gets duplicated in the presence of a B chromosome [ 73 ]. It was for the first time that such a phenomenon has been reported in plants, earlier, the presence of macrospermatids (> diploid chromosome number) in B containing individuals of a grasshopper Dichroplus pratensis were observed [ 85 ].…”