Asexual Reproduction Part 1 PBG 430 Asexual reproduction

Asexual Reproduction – Part 1 PBG 430

Asexual reproduction: Overview

Example #1: Garlic (Allium sativum) § “As far as we know, garlic in cultivation throughout history has only been propagated asexually by way of vegetative cloves, bulbs, and bulbils (or topsets), not from seed. These asexually propagated, genetically distinct selections of garlic we cultivate are more generally called "clones". Yet this asexual lifestyle of cultivated garlic forgoes the possibility of combining traits proffered by interpollinating diverse parental stocks. ” ~Philipp Simon, USDA/ARS, Madison, Wisconsin, USA Plant Garlic (Allium sativum) Formula 2 n = 2 x = 16 Genome size 15, 900 Mb Approximate number of genes Unknown Genome sequence Not yet sequenced Pollination biology Asexual reproduction Center of origin Asia / Southwestern Siberia

Asexual reproduction § The clone is immortal…but, what if you want to change it? § Perspectives on breeding garlic (as an example): a) Induced mutation b) Transgenics c) Gene editing d) Restore sexual fertility

Example #2: Aspen (Populus tremuloides) § The “Pando clone” § Aspen is a dioecious species – the Pando clone is one big male § Single genotype forest § The world's heaviest living thing (roughly 6 million kilograms) § Apparently it is one of the world’s most ancient organisms § 1 seed originally and now it has 47, 000 genetically identical ramets § Currently is threatened! § Mule deer herbivory Plant Aspen (Populus tremuloides) Formula 2 n = 2 x = 38 (2 n = 3 x = 57 does exist) Genome size 550 Mb Approximate number of genes 38, 000 Genome sequence Not yet sequenced Pollination biology Asexual / obligate outcross (dioecious) Center of origin North America

Example #3: Apple (Malus pumila/Malus domestica) § Sexual and asexual reproduction § ‘Fuji’ variety § Resulted from the cross between ‘Red Delicious’ and ‘Virginia Ralls Janet’ in the early 1900 s. Plant § ‘Red Delicious’ variety § Resulted from somatic mutations in the ‘Delicious’ variety. § Many subsequent mutations were selected § Developed in the late 1800 s § ‘Delicious’ variety § Developed in the late 1800 s Apple (Malus pumila / Malus domestica) Formula 2 n = 2 x = 34 Genome size 643 Mb Approximate number of genes 42, 000 Genome sequence Peace et al. (2019) Pollination biology Asexual / Outcrossing (monoeicious, selfincompatibility) Center of origin Central Asia / Kazakhstan

Apomixis

Seeds without sex: Apomixis § Apomixis = parthenogenesis: the development of an egg without fertilization § Versus Parthenocarpy = development of fruits (seedless) without fertilization § Apomixis allows for seed propagation of heterozygotes that are genetically identical from generation to generation. § The two types: § Obligate apomict a species can only reproduce apomictically § Facultative apomict a species can either reproduce apomictically or sexually

How prevalent is apomixis? § Monocots and dicots § Approximately 400 species; 40 families; § Common in Poaceae, Asteraceae, Rosaceae § Examples: § Tripsacum § Poa § Taraxacum § Rubus

Apomixis: Mechanisms § How does an apomictic egg become diploid? § Mechanisms: § No (or altered) meiosis to produce unreduced (2 n) female gametophyte (embryo sac) § Development of an embryo without fertilization § Development of an endosperm with or without fertilization. § Pseudogamy = fertilization of central cell but not egg

Apomixis: Mechanisms continued § Sexual reproduction (review) (number 1 in this figure) § Apomictic seeds can form through: § Sporophytic apomixis (number 2) § 2 n embryo sac develops from a somatic ovule cell that is not the megaspore mother cell § Apospory (number 3) § 2 n somatic cell surrounding the embryo sac differentiates into an embryo § Diplospory (number 4) § 2 n embryo sac develops from megaspore mother cell 1 Hand Koltunow. 2014. The Genetic Control of Apomixis: Asexual Seed Formation. Genetics 197(2): 441 -450 2 3 4

Genetic basis of Apomixis § While the genetic basis of apomixis is unknown, many loci and candidate genes have been identified. § Considerable effort in breeding for apomixis, with little success. § What about Epigenetics? § Heritable phenotype changes not reflected in DNA sequences § “Our results highlight the potential of heritable DNA methylation variation to contribute to population differentiation along ecological gradients. ” (Preite et al. , 2015)

Apomixis: Implications § Economic implications of apomixis § Hybrid seed production? § Transgenic apomict escape? § Evolutionary implications of apomixis § Obligate § Facultative

By now you should be able to… 1. Define asexual reproduction in plants and describe its genetic implications. § Asexual = no sex = no fertilization; no fusion of female and male gametes § Asexual reproduction = clonal propagation = progeny inherits parental genome 2. Give three examples of plants that can reproduce asexually. Identify the tissue used for propagation – stems, roots, leaf buds, etc. 3. Define apomixis and describe three main types of apomixis. For each type, you should know: § From which cell does the embryo come from? § How does the embryo become 2 n? • Was there a bypass of meiosis? • Was fertilization of the central cell required for the apomictic seed formation? § How is the endosperm developed? 4. Describe commercial advantages and disadvantages of apomixis. 5. Discuss the advantages and disadvantages of asexual reproduction to plant evolution and breeding (e. g. think about what you can do to change a phenotype in garlic).