Plant Reproduction Gene transfer, asexual reproduction, sexual reproduction, and apomixis viachicago.wordpress.com tinyfarmblog.com birdsandbloomsblog.com

Asexual reproduction The clone is immortal Example: 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.” Source:

Asexual reproduction The clone is immortal Example: Populus tremuloides The world's heaviest living thing 1 clone in the Wasatch Mountains of Utah 47,000 stems of genetically identical aspen trees Total weight: 6 million kilograms Aspen is dioecious species - this clone is one big male source:

Sexual reproduction Advantages > disadvantages Advantages: Genetic variation: Allele exchange via cross-pollination New combinations of alleles via meiosis Purge deleterious mutations Stay ahead in the host-pathogen “arms race” Potential adaptation to a changing climate

Sexual reproduction Advantages > disadvantages Disadvantages: In a dioecious species, half the reproductive effort is wasted in producing males Meiosis produces some "unfit" combinations of alleles Cross-pollinated plants may be subject to environmental conditions unfavorable to pollination Finding a mate!

Alternation of generations In plants there is an alternation of the gametophytic (n) and sporophytic (2n) generations (Gametophytic = haploid)

Alternation of generations The sporophytic generation may be diploid (2n = 2x) or polyploid (2n = _x) VAVAVAVA VAVAVBVBVAVAVBVB VAVAVBVBVDVDVAVAVBVBVDVD 2n = 2x = 14 30,000 genes 2n = 4x = 28 60,000 genes 2n = 6x = 42 90,000 genes 1 pair homologous chromosomes 0 sets of homoeologous chromosomes AA 2 pairs of homologous chromosomes 2 sets of homoeologous chromosomes AABB 3 pairs of homologous chromosomes 3 sets of homoeologous chromosomes AABBDD A A A B B A A B B D D

Angiosperm reproductive organs and gamete formation Development of the female gametophyte Reproductive structure: Ovule(s), style, stigma

Megaspore mother cell (MMC) MMC undergoes meiosis Of 4 megaspores produced 1 survives (most species) Three post-meiotic mitoses Angiosperm reproductive organs and gamete formation Development of the female gametophyte 1 2 3

The 8-nucleate embryo sac (1 egg, 2 synergids, 2 primary endosperm nuclei, 3 antipodals) Angiosperm reproductive organs and gamete formation Development of the female gametophyte Source: yougems.reflectionsinfos.comlima.ohio-state.edu

Pollinator attraction: Petals, nectaries, etc. Angiosperm reproductive organs and gamete formation Development of the female gametophyte Source: arizonabeetlesbugsbirdsandmore.blogspot.com

Angiosperm reproductive organs and gamete formation Development of the male gametophyte Reproductive structures: Anthers; pollen within anthers

Angiosperm reproductive organs and gamete formation Development of the male gametophyte Pollen mother cell (PMC) PMC undergoes meiosis Meiosis gives a tetrad of microspores Meiosis 1Meiosis 2 Note, this is different than ♀

Angiosperm reproductive organs and gamete formation Development of the male gametophyte The first mitosis gives vegetative and generative nuclei; at the second mitotic division, the generative nucleus gives 2 sperms. mitosis

The pollen pathway and double fertilization The stigma is the site of pollen recognition Pollen germinates and the vegetative (tube nucleus) grows through the style to the ovule The two sperm use the tube as conduit

The pollen pathway and double fertilization Double fertilization: One sperm fertilizes the egg to give the 2n embryo, the other fertilizes the polar nuclei to give the 3n endosperm antipodals egg synergids polar nuclei ♀♀ ♀ ♀ ♀ ♀ ♂ ♂ 3n endosperm 2n embryo

The pollen pathway and double fertilization After double fertilization, there are at least four independent and genetically distinct generations coexisting in the seed: maternal sporophyte diploid tissue maternal gametophyte haploid tissue offspring sporophyte diploid tissue fusion of male (1) and female (2) gametophyte to form triploid tissue

A review… MMC V v PMC V v v v V V V V v v v v V V V V v v V V V V V V V v v v v v v v v v v v v V V V V V V v sporophytic generation

A review of sexual reproduction in an F1 plant, heterozygous for the Vv locus Vv VVVVv v vv += V V V V V V V V V V += v v V V V V V V V V V v V v += v v v v v v v v v v + = V V v v v v v v v v v V v V Note: At this point in the figure, the antipodals and synergids are deleted and only the fertilized endosperm nuclei (now 3n) and fertilized egg (now 2n) are shown. Only the fertilized egg is carried to the Punnett square.

Seeds without sex: Apomixis Apomixis involves parthenogenesis – development of an egg without fertilization, as opposed to parthenocarpy – development of fruits (seedless) without fertilization Implication - Apomixis allows for seed propagation of a heterozygote – genetically identical from generation to generation. 1. Obligate: 2. Facultative:

Seeds without sex: Apomixis Source: Grossniklaus et al The Plant Cell.

Economic implications of apomixis Why would apomixis be, or not be, of commercial interest to purveyors of hybrid seeds? If transgenic apomicts are developed and released, and if apomixis is dominant, it will be necessary to control pollen flow – if not, there will be frozen gene pools Evolutionary implications of apomixis Obligate vs. facultative Seeds without sex: Apomixis

Prevalence 400 species ; 40 families; Common in Poaceae, Asteraceae, Rosaceae Examples Tripsacum Poa pratensis Pennisetum Dandelion (Taraxacum spp) Rubus Seeds without sex: Apomixis

How it works no (or altered) meiosis to produce unreduced female gametophyte (embryo sac) no fertilization – but do get autonomous embryo formation may have autonomous endosperm development, or endosperm development may be triggered by fertilization. Most cases it is triggered by fertilization (pseudogamy = fertilization of central cell) Seeds without sex: Apomixis

Genetic basis Reported to be anything from one gene to many Lots of breeding effort; little success Perhaps all genes related to megasporogenesis! Seeds without sex: Apomixis

The Rubus armeniacus case study Himalayan (Armenian) blackberry

Is the Himalayan blackberry the perfect weed? promo.idahopotato.com Designing the perfect invasive AttributeDescription FlowerHermaphroditic Pollination biologySelf and outcross ApomixisFacultative SeedsSmall and numerous Vegetative propagationYes Ploidy levelPolyploid ProtectionThorns AttractionTasty fruit Was Luther Burbank the “father” of this perfect weed?