The Hows and Whys of Cytoplasmic Inheritance in seed

plantsH. Lloyd MogensenNOT ALL INHERITED CHARACTERS ARE DETERMINED BY GENES

LOCATED IN THE NUCLEUS

WHAT IS CYTOPLASMIC INHERITANCE

Some genes are passed on from parent to offspring without ever being part of a nuclear chromosome. Where are these genes found, and how does this non-nuclear inheritance occur?

Cytoplasmic inheritance is transmission of genes that occur outside the nucleus

Since these genes are present outside the nucleus, in the cytoplasm, they are called ‘PLASMAGENES’

Mostly occurs in eukaryotes through cytoplasmic organelles – chloroplast and mitochondria or from cellular parasites like viruses or bacteria

Also called extra-nuclear inheritance

CHARACTERISTICS OF CYTOPLASMIC INHERITANCE

(1) Reciprocal differences

(2) Lack of Segregation

CHARACTERISTICS OF CYTOPLASMIC INHERITANCE

LEAF VARIEGATION

IN Mirabilis jalapa

LEAF VARIEGATION IN Mirabilis jalapa

• Earliest and best known example discovered by Correns

• Phenotype of the progeny always resembled the female parent but male made no contribution at all to the character

• Explanation: genes concerned for are located in the chloroplasts within the cytoplasm and not in the nucleus and are therefore transmitted only through female plant (maternal inheritance).

• Chloroplasts have no regular means of distribution, i.e. equal distribution in daughter cells may not occur. So, some branches may appear mosaic while others green or white.

HOMOPLASMY AND HETEROPLASMY

• The number of copies of organelle genome per organelle can vary from one to many.

• A cell or organism in which all copies of an organelle gene are the same is called Homoplasmic or is said to exhibit Homoplasmy

• A cell or organism in which not all copies of an organelle gene are the same is called Heteroplasmic or is said to exhibit Heteroplasmy.

• Heteroplasmic cell can produce heteroplasmic and homoplasmic descendants.

HOMOPLASMIC

HETEROPLASMIC

REPLICATIVE SEGREGATION

Random segregation of organelles during replication –

replicative segregation

CYTOPLASMIC INHERITANCE

IN ANGIOSPERMS

MECHANISMS OF CYTOPLASMIC INHERITANCE IN ANGIOSPERMS

(A)Exclusion of plastids from the generative cell

Example: Members of the family Orchidaceae

MECHANISMS CONTD.

MECHANISMS CONTD.

MECHANISMS CONTD.

(B) Loss of cytoplasmic organelles from generative cell

Examples; Nicotiana, Plumbago, Solanum

• Degeneration and/or modification of plastids.• Mostly does not occur until the division of generative

cell within the pollen tube.• Mitochondria and chloroplasts enclosed within ER -

derived autophagosomes which fuse with lysosome (autophagolysosome) – digestion of organelles.

• Since digestion is impartial, mitochondria may be undergoing preferential replication. Mitochondria thus maintains its number in some species.

MECHANISMS CONTD.

(C) Loss of cytoplasmic organelles from sperm cells

Example: Barley (Hordeum vulgare)• In an ultrastructural study of barley sperm cell maturation within pollen indicates that the mitochondria within the sperm cell is reduced by 50% from the time the sperm cells are formed until pollen maturity at anthesis.

• Organelle loss results from cytoplasmic projection formations that are ultimately discarded.

MECHANISMS CONTD.

(D) Exclusion of Male cytoplasm during gametic fusion

Example: Barley, Wheat

• In barley, large number of mitochondria are present in the sperm cell upto the time of fertilization.

• Examination of embryo sacs immediately after fertilization when sperm nuclei are positioned at the synergids, reveals presence of cytoplasmic body appressed to the egg cell where the nucleus is likely to enter the egg.

MECHANISMS CONTD.

• So what did the cytoplasmic body contain? 7 dictyosomes, 59 mitochondria, 3 plastids and a

large membrane-less vacuole but no nucleus.• Essentially the entire cytoplasm is excluded during

syngamy in Barley.

MECHANISMS CONTD.

(E) Degradation of organelle DNA within generative and/or sperm cells – Examples:

Wheat, Barley, Rice• Sometimes, structurally intact plastids and mitochondria

may survive and even be transmitted to egg cell, but they may lack DNA or possess DNA that is greatly modified such that it is not heritable.

• Organelle DNA modification may take place during pollen maturation and may thus lead to suppression of male cytoplasmic inheritance.

• Some of the modifications include lack of normal plastid genome products (23s and 16s ribosomal RNA and large subunit of Rubisco enzyme) or deletions in the chloroplast DNA.

MICROSCOPIC TECHNIQUES TO STUDY CYTOPLASMIC INHERITANCE

• Disappearance of plastid and mitochondrial DNA (nucleoid) from the generative cells during pollen maturation was studied by staining with DNA-specific fluorochrome DAPI

DAPI stained only the generative cell,

organelle nucleoids absent

DAPI stained

generative cell

DAPI stained organelle nucleoids

MOLECULAR TECHNIQUES TO STUDY CYTOPLASMIC INHERITANCE

Question – Does the absence of DAPI detectable cytoplasmic DNA really mean that, indeed, the DNA has been completely deleted??

SNAPDRAGON

Maternal plastid

inheritance

ALFALFABiparental

inheritanc

eSouthern Blot Hybridization with a probe specific for plastid

DNA (rbcL gene, which codes for large subunit of Rubisco)

Plastid specific probe hybridized with Alfalfa DNA

Plastid specific probe did not hybridize with

Snapdragon DNA*This experiment validated DAPI staining results

ENZYMATIC STUDIES FOR CYTOPLASMIC INHERITANCE

• If cytoplasmic DNA loss occurs during pollen maturation in plants displaying maternal cytoplasmic inheritance, perhaps the loss is due to specific nucleases within the pollen of these species.

• Nuclease C (Ca2+ dependent nuclease purified from Chlamydomonas reinhardtii ) is also found in plants known to inherit plastids maternally.

• Two plants – a) Lilium longiforum (showing maternal plastid inheritance) and b) Pelargonium zonale (showing biparental inheritance) were studied for the presence of nuclease C by SDS_PAGE. Whereas, nucleases were found in the pollen protein extracts of L. longiforum, they were absent from the pollen or ovary extracts of P. zonale.

• It was also found that Nuclease C content was high in the stamens of Mirabilis jalapa.

CYTOPLASMIC INHERITANCE

IN GYMNOSPERMS

LIFE CYCLE OF GYMNOSPERMS

Unlike angiosperms,

double fertilization

does not occur!!

MECHANISMS OF CYTOPLASMIC INHERITANCE IN GYMNOSPERMS(I) Paternal plastids and maternal mitochondria (Example: Douglas Fir)

MECHANISMS CONTD.

(II) Paternal plastids and paternal mitochondria (Example: Biota orientalis)

(III) Maternal plastids and maternal mitochondria (Example: Ephedra distachya)

MECHANISMS CONTD.

DISCUSSION

Why is the uniparental inheritance so prevalently maternal?? Why is the female cytoplasm not eliminated?

It would be more disadvantageous if the more aggressive and deleterious types of plastids were transmitted through pollen because they would be more widespread than mutations occurring in the female plant. Exception – Alfalfa.

Maternal cytoplasmic inheritance may have evolved as a mechanism that prevents foreign or pathogenic DNA from entering the egg (alien endosymbionts plastids).

Reduction in sperm cytoplasm to facilitate its movement within the pollen tube could be a reason.

Biology is a science of exceptions!!

SIGNIFICANCE

• Development of cytoplasmic male sterility and thus manifestation of heterosis

• New varieties generated by mutations using organelle specific mutagens (eg. Fluorodeoxyuridine for chloroplast DNA and Manganese ions for mitochondrial DNA) (Ralph Bock (2001) Transgenic Plastids in basic research and Plant Biotechnology. J. Mol. Bio 312, 425-438)

• Control of gene flow by Transgenics

REFERENCES

i. H. Llyod Mogensen (1996) The Hows and Whys of Cytoplasmic Inheritance in Seed Plants, American Journal of Botany 83(3)

ii. Noriko Nagata (2010) Mechanisms for independent cytoplasmic inheritance of mitochondria and plastids in angiosperms, J. Plant Res 123:193-199

iii. Cytoplasmic inheritance and Evolution of Organelle Genomes, Mark F. Sanders and John L. Bowman (Slideshare.com)