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

Abstract

Summary Heracleum pinnatum C. B. Clarke (Apiaceae) collected from the cold deserts of Ladakh Himalaya, Jammu and Kashmir, India, is cytologically analyzed for the first time, revealing a diploid chromosome count of 2n=22. During meiosis, the majority of the pollen mother cells (PMCs) exhibited 11 bivalents, equal segregation of chromosomes during anaphases, regular tetrads, and normal-sized pollen grain formation. Occasionally, two proximate PMCs fused during the early stages of prophase-I and resulted in the formation of syncytes. The frequency of such syncytic meiocytes was low (3.72-3.96%) but these could be easily detected in the preparations due to their large size compared to typical PMCs. Also, the syncytes depicted a meiotic chromosome count of 2n= 44 as confirmed from the presence of 22 normal bivalents. Further, meiotic course in such tetraploid PMCs was also observed to be perfectly regular, leading to the formation of 2n or larger-sized pollen grains that are almost double the size of typical n pollen grains. Such larger-sized 2n pollen grains could also be differentiated on the basis of their shape. Additionally, a significant frequency of PMCs also showed the phenomenon of cytomixis involving transfer of chromatin material resulting in aneuploid meiocytes. Whole chromatin transfer during cytomixis among neighboring PMCs at early stage of meiosis-I lead to the formation of PMCs with double the chromatin material. Syncytes resulted as a consequence of fusion of meiocytes during the early stages of meiosis-I could be attributed to low temperature stress conditions prevailing at the time when the plants enter the flowering stage. It is quite possible that such apparently fertile 2n pollen grains originating from syncytes might play a role in the origin of polyploids in the species.

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).…”

“…However, Ramsey and Schemske (1998) emphasized that pairing of chromosomes is not always a good indicator of nature of polyploidy. Intraspecific euploids in plants were thought to have been evolved either through somatic gametic duplication (somatic polyploidization) or through the formation of diplogametes involving different mechanisms such as meiotic restitution (Bretagnolle and Thompson 1995, Andreuzza and Siddiqi 2008, Dewitte et al 2010, abnormal spindle activity (Rana et al 2013) or as a product of pre-meiotic fusion of meiocytes/whole chromatin transfer during cytomixis (Singhal and Kumar 2008, Kim et al 2009, Singhal et al 2011, 2016, De Storme and Geelen 2013. Presence of chromatin transfer among meiocytes and cell fusion resulting into syncytes and polyploid meiocytes in some of the 4x populations of the species might have played a role in the production of unreduced gametes which have facilitated the production of polyploids.…”

Meiotic study in a completely male sterile wild plant of <i>Deyeuxia scabrescens</i> from Kullu District, Himachal Pradesh (Northwest Himalayas), India

“…Presence of sterile/non-viable pollen grains in D. scabrescens seem to have been resulted due to aberrant meiotic divisions in pollen mother cells caused due to cytomixis and cell fusion as advocated by many workers in other plants (Li et al 2005;Singhal et al 2011;2016;Lattoo et al 2006;Liu et al 2012). However, the total pollen sterility in the presently analyzed accession could be attributed to failure of starch biosynthesis in mature pollen grains which seems to be under the control of some genetic factors as reported earlier in male sterile clones of some grasses (Datta et al 2001(Datta et al , 2002Jain et al 2007;Kong et al 2007) and therefore there is a great need to studied this on molecular basis which is not possible in the present study.…”

Karyotype and Chromosome Behavior Analyses in Three Regions of the Indomalayan Realm

“…The predominant geographical features of North India (including Punjab) are the following three regions: the Indo-Gangetic plain, which spans the states of Punjab, Haryana, Uttar Pradesh, and Bihar (Gupta et al 2017a). The Himalayas, which span three states, Uttarakhand, Himachal Pradesh, and Jammu and Kashmir, contain many endemic species: Impatiens devendrae Pusalkar and Meconopsis aculeata Royle (Kumar et al 2016) in Uttarakhand, some members of subclass Gamopetalae (Gupta et al 2016a), Achillea millefolium L. (Kaur et al 2017), Astragalus rhizanthus Royle ex Benth. , Sedum oreades (Decne) R. Hamet , species of the genus Clematis L. (Kumar et al 2017), Melica persica Kunth (Kumari and Saggoo 2016), Eremurus himalaicus Baker (Kumari and Saggoo), Angelica glauca Edgew (Rani et al 2016), Pteris cretica L. (Saggoo and Kaur 2017a), Adiantum lunulatum Burm (Saggoo and Kaur 2017b), Anthoxanthum odoratum L. (Singhal and Kumari 2017), some dicots (Singhal et al 2016b), and Neurobasis chinensis L. (Walia et al 2016) in Himachal Pradesh.…”

“…Species endemic to Jammu and Kashmir include Angelica glauca Edgew. (Rani et al 2016), Lavatera cachemiriana Cambess (Saggoo et al 2017), Heracleum pinnatum C. B. Clarke (Singhal et al 2016a), Tanacetum artemisioides Sch. Bip.…”