Cytomictic Anomalous Male Meiosis and 2nPollen Grain Formation inMertensia echioidesBenth. (Boraginaceae) from Kashmir Himalaya

Abstract

Presently Mertensia echioides Benth. (Boraginaceae) collected from Kashmir Himalaya, India, is cytologically analyzed for the first time revealing 2n = 2x = 24 (diploid). Interestingly we found 4.3–6.2% syncytic meiocytes/PMCs with 2n = 4x = 48 (tetraploid) in addition to normal meiocytes (2n = 24) during male meiosis. These comparatively larger PMCs (pollen mother cells) lead to the formation of fertile giant 2n pollen grains. A frequency of 6.4–13.3% PMCs shows transfer of chromatin material at prophase-I and, therefore, results in aneuploid meiocytes. Whole chromatin transfer by the process of cytomixis could also have led to the formation of tetraploid cells. Translocation heterozygosity is also evident in the form of multivalents in 12–17% diploid (2x) meiocytes at diakinesis and metaphase-I and is reported for the first time in this species. The syncytes formed depict open chain hexavalent and quadrivalent formation in the three populations with different frequencies. Moreover chromatin stickiness at metaphase-I is observed in 45% of PMCs in population-1 (P-1). Syncyte or unreduced PMC formation leading to unreduced fertile gametes is here speculated to act as a possible way out for infraspecific polyploidization in the species.

Syncyte and ‘2<i>n</i>’ Pollen Grain Formation in <i>Heracleum pinnatum</i>: A Possible Mechanism for the Origin of Intraspecific Polyploids

“…While exploring the cytomorphological diversity in the flowering herbs of cold desert regions of the Ladakh division, we have recorded the presence of syncyte meiocytes through the process of cytomixis involving whole chromatin transfer and direct cell fusion during the early stages of prophase in plants of H. pinnatum. A similar type of cell fusion and transfer of genetic material during cytomixis resulting in syncyte PMCs has also been reported in the wild plants of Meconopsis aculeata (Singhal and Kumar 2008), Chrysanthemum (Kim et al 2009), Lindelofia longiflora (Singhal et al 2011), and Mertensia echioides (Malik et al 2014). Such meiocytes with a tetraploid chromosome count of n=22 showed a perfect 22 bivalents during Metaphase-I, regular meiosis, and normal sporads.…”

“…Such a low temperature might have affected the meiotic process, particularly at the premeiotic stage, causing cell fusion and complete transfer of chromatin material during cytomixis. The impact of genetic and environmental (low temperature) factors on meiosis and subsequent formation of unreduced or 2n gametes have also been documented in several plant species by various researchers (Bretagnolle and Thompson 1995, Ghaffari 2006, Singhal and Kumar 2008, Singhal et al 2011, Malik et al 2014). …”

“…Also, the larger sized 2n pollen grains are well stained and appeared to be fertile. It is quite possible that these might play a role in the origin of intraspecific polyploids in the species as advocated by Price (1956) in Eriannthus and Saccharum, Kim et al (2009) in Chrysanthemum, Singhal et al (2011) in Lindelofia longiflora, and Malik et al (2014) in Mertensia echioides. Production of such 2n pollen grains through sexual polyploidization (through syncytic meiocytes) certainly has the advantage over synthetic PPDS (polyploids produced through the use of spindle inhibitors).…”

“…Ever since the first report of syncyte formation through cell fusion in Lactuca sativa by Gates and Rees (1921), a number of cases have been reported (Levan 1941, Price 1956, Katayama 1964, Mehra and Kalia 1973, Sarbhoy 1980, Patra et al 1987, Padmaja 1988, Caetano-Pereira et al 1999, Boldrini and Pagliarini 2006, Risso-Pascotto et al 2006, Singhal and Kumar 2008, Kim et al 2009, Kumar et al 2010, MendesBonato et al 2011, Singhal et al 2011, Malik et al 2014. As defined by Levan, syncyte formation involves the fusion of two or more PMCs usually during the early prophase stages of the first meiotic division, giving rise to 2n gametes after meiosis (Sarbhoy 1980).…”

Intraspecific Euploidy (2<i>x</i>, 4<i>x</i>) in <i>Primula denticulata</i> Sm. from North West Himalayas in India

“…However, in one of the 4x accession, a few meiocytes showed the phenomenon of cytomixis and cell fusion resulting into pollen malformation in the form of sterile and unequal-sized pollen grains. Such an effect of cytomixis and cell fusion on pollen viability and pollen size variation has been reported in number of cases such as, Meconopsis aculeata Kumar 2008), Chrysanthemum (Kiṁ et al 2009), Lindelofia longiflora (Singhal et al 2011), Mertensia echioides (Malik et al 2014) and Heracleum pinnatum (Singhal et al 2016). Polyploidy has been considered to be an important feature of chromosome evolution in plants (Ramsey and Schemske 1998).…”