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Fall 2015 There's Much Left ot Learn: 's Chromosomes W. John Hayden University of Richmond, [email protected]

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Recommended Citation Hayden, W. John. "There's Much Left ot Learn: Clethra's Chromosomes. " Sempervirens, Fall 2015, 4-5.

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By W. John Hayden, Botany Chair

any would argue that Chromosomes were of some species have 18 right stage (late prophase I chromosomes, genes, interest to biologists, how- chromosomes per cell, or metaphase I of meiosis), and DNA form the ineluc- ever, long before anyone others 36, still others 54, all the chromosomes are table essence of modern ever dreamed of DNA se- or 72, or 90. (These are all tightly paired, so close biology. Not only do these quencing or reading entire nongamete-related counts together, in fact, that each fundamental components genomes. Chromosomes from diploid cells.) The pair appears through the of living cells provide mo- have been an object of realization that counts microscope as a single ment-to-moment instruc- intense scrutiny since the from these related species body. A count made at this tions by which cells carry details of cell division— were all multiples of the stage is called a haploid (n) out basic life processes, mitosis and meiosis—were number nine led to the count because the number they also control inher- elucidated in the late 19th concept of polyploidy, the of chromosomes observed itance of characteristics century. An early insight occurrence of multiple full will match the reduced from one generation to connecting chromosomes sets of chromosomes in number that will be found the next. These essential with the mechanism of series of related species. in the gametes that soon functions of DNA stem heredity derived from the Chromosome counts form. from its repetitive struc- observation that gametes are made from cells caught Available information ture. Hugely long DNA (sperm, eggs) of and in the act of cell division. indicates that the genus molecules are built from animals contain half the A commonly used source Clethra , in which Sweet just four components, number of chromosomes is the actively growing root Pepperbush, the 2015 referenced by their single- present in other, nonre- tip, with cells caught in VNPS Wildfl ower of the letter abbreviations, A, C, productive cells of the late prophase or metaphase Year, is classifi ed, contains G, and T. It is the specifi c same species. Although the of mitosis (ordinary cell a polyploid series (see sequence of these As, Cs, actual number of chro- division). Root cells have summary box). So far, Gs, and Ts that constitutes mosomes present varies two sets of chromosomes, four species, C. alexandri , the coded information of from species to species, the one full set received from C. arborea , C. lanata , DNA. Moreover, molecu- general terms haploid and each parent; a count made and C. pringlei , are lar biologists have deter- diploid refer, respectively, from a root tip thus yields reported to have a diploid mined that this genetic to this distinction between what is called a diploid chromosome number of code is universal, i.e., reproductive and vegeta- (2n) count. Alternatively, 16. ’s two species, petunias, earthworms, tive cells; haploid gametes chromosomes can also be C. acuminata and C. chimpanzees, and every- have only a single set of counted from anther cells alnifolia , have a diploid thing else spell out their chromosomes, whereas undergoing meiosis before count of 32. And Clethra genetic instructions using diploid cells have two. pollen formation. (Meiosis barbinervis , from Japan the same three-letters-at- An early publication reduces the chromosome and sometimes cultivated a-time code words. We are (Winge 1917) established number by half, a process in botanical gardens, is now well into the genome that chromosome numbers that precedes gamete reported to have a diploid era, in which biologists in closely related plants formation. In most plants, count of 80. seek meaning by reading, often follow predictable including Clethra , sperm Clearly, for the for many organisms, the patterns. For example, cells do not form until genus Clethra , the base codes contained in the the chromosome number after pollination, when the chromosome number long strings of As, Cs, Gs, in species of the genus pollen grain has formed (often designated as x) is and Ts that encompass the Chrysanthemum has been a pollen tube inside the eight, and each species totality of their DNA. shown to vary widely; style.) If observed at the in the series has some Sempervirens , Fall 2015 5

multiple of eight for its

d e f g e h i j f k f l f k m h f n e o l p e q r s t u v w x

total chromosome count in

y z n k k { j |

vegetative cells.

~  €  ~ } ‚ ƒ „ †  ‡ ˆ ~ ‡ † ‰ Š ‹ Œ ‰ ƒ Š ‹  Ž At this point the } terminology gets a little C. arborea, from Madeira (Hagerup 1928) tricky, because the word C. alexandri, from Jamaica (Judd 2008) diploid can be used in two C. lanata, from Mexico (Kyhos 1965) contexts, with different C. pringlei, from Mexico (Reed 2005)

meanings. As mentioned

 ‡   ‘  €  ~ } ‚ ’ „ †  ‡ ˆ ~ ‡ † ‰ Š ‹  Ž ‰ ƒ Š ‹ “ ƒ above, a diploid cell, in a life-cycle frame C. acuminata, from eastern North America (Tanaka & Oginuma 1980) C. alnifolia,

of reference, denotes from eastern North America (Tanaka & Oginuma 1980)

‡ ˆ ‘  €  ~ } ‚  ” „ †  ‡ ˆ ~ ‡ † ‰ Š ‹ ’ ” ‰ ƒ Š ‹ Œ ” a nonreproductive cell } containing twice as many C. barbinervis, from Japan (Tanaka & Oginuma 1980) chromosomes as found Illustration of Clethra alnifolia by Lara Call Gastinger from Flora of Virginia. © Flora of Virginia Project in pollen and eggs. But the concept of diploid can be extended to apply to have 16 chromosomes. Little more can be we know only a little, species at the base of a Clethra acuminata is also inferred from the data and we don’t know a lot. polyploidy series. considered a tetraploid at hand because, of the But the little that we do A diploid species species. But C. barbinervis 65 or so known species know about chromosome refers to a species that is a decaploid species, of Clethra , chromosome numbers in Clethra hints has just two sets of with 10 sets of chromo- counts have been pub- at a rich story waiting to

chromosomes in vegetative somes (= 80) in nonrepro- lished only for the seven be uncovered. v

\ ] ^ _ ` ] a _ ^ b \ ] ^ c cells; similarly, a triploid ductive cells and 5 sets of mentioned in the summary [ species would have three chromosomes (= 40) in box. In other words, nearly sets, and a tetraploid gametes. 90 percent of the species Hagerup, O. 1928. Morpholog- ical and cytological studies species would have four, Generally speaking, of Clethra are unknown in of Bicornes. Dansk Bot. etc. Clethra arborea , for polyploid series start at terms of this fundamental Ark. 6: 1–27. example, is considered the diploid level, and, aspect of their biology. Judd, W. S. 2008. Scientifi c to be a diploid species by various mechanisms, We don’t know how many note: chromosome number of Clethra alexandri Griseb. because cells of its roots, higher chromosome counts of the 58 species without (). Castanea 73: stems, and leaves contain arise over time. Almost published chromosome 333–334. 16 chromosomes (two sets certainly, then, ancestral counts will prove to be Kyhos, D. W. 1965. Docu- of eight). (These would Clethra , like modern C. diploid species. We don’t mented chromosome num- bers of plants. Madroño 18: also be diploid cells in the arborea , was diploid, 2n = know if tetraploid species 122–126. life-cycle context, and its 2x = 16. It should also be are common or whether C. Reed, S. M. 2005. Cytolog- eggs and sperm would certain that the ancestry alnifolia and C. acuminata ical analysis of a Clethra alnifolia ‘Hokie Pink’ × C. be haploid, with one set, of C. alnifolia and C. are the only two. And we pringlei hybrid. HortScience eight chromosomes, each.) acuminata includes at least don’t know if there are 40: 339–342. Moving up the polyploid one jump, from the diploid any species of Clethra Tanaka, R., & K. Oginuma. series, C. alnifolia is a to the tetraploid level. alive today that bridge the 1980. Karyomorphological studies in Clethra barbi- tetraploid species; its And who knows what gap between tetraploids nervis and two allied nonreproductive cells complex path may have (2n = 4x = 32) and the species. J. Japanese Bot. 55: (diploid in the life-cycle occurred in the ancestry of decaploid (2n = 10x = 80) 65–72. context) contain 32 decaploid C. barbinervis . status of C. barbinervis. Winge, Ö. 1917. The chromo somes: their chromo somes (four But whatever the details, Nor do we know whether number and general im- multiples of the base the trend was from fewer any living Clethra species portance. Compt. Rend. chromosome set of eight), to more whole sets of has a chromosome count Trav. Lab. Carlsberg 13: while sperm and eggs chromosomes. greater than 80. Clearly, 131–275.